What are the ways to add new weapon allowances to Decisive Strike/Flurry of Blows?

Question

For thematic reasons, I am wanting to make a Zatoichi-style character based on using Quick Draw and Decisive Strike monk alternate feature from the PHB2. The only catch is that Decisive Strike says:

. . . make one attack with an unarmed strike or a special monk weapon . . .

What are all of the official ways to add a weapon to the list of monk special weapons? I have found only one in the Shou Disciple prestige class (Unapproachable East, p.32), but doing so would mean the earliest the character could use decisive strike with a katana would be level 10. I would like to be doing it no later than level 5 if such an option exists.

Note that, for the purposes of this question, anything that works for "Flurry of Blows" can be interchanged with "Decisive Strike" (e.g. the "Martial Flurry" ability of a Shou Disciple gains ability to use other weapons with "Flurry of Blows," but that is fine).

Answer 1

Feats

Eberron Campaign Setting has a few of feats for this. Why they’re found there of all places, I certainly don’t know, but it is what it is. Secrets of Sarlona has another (more sensible), and Dragon Compendium a couple more (why not?). HeyICanChan points out that Dragon vol. 352 has another feat, which hits three exotic weapons introduced in that issue, none of which are worth using (I guess the kaua’koi has some benefit against DR?). None of them cover a katana (or masterwork bastard sword).

There is the Flowing Blade¹ feat from the Dragonshards article “Blades of the Quori,” which makes a soulknife’s mind blade a special monk weapon. Complete Psionic has a number of feats for changing a mind blade into various exotic weapons... but a katana or bastard sword isn’t one of them. Secrets of Sarlona adds another mind blade shape feat—a monk’s spade—but no katana. To get a katana we have to turn to Dragon vol. 347 and its Reshape Mind Blade¹ feat, which is what Complete Psionic should have printed in the first place. But that forces you to get, and use, a mind blade, which probably doesn’t work for you.

Feat	Flurry with...	Source
Double Steel Strike	Two-Bladed Sword	Eberron Campaign Setting
Flowing Blade1	Mind Blade	“Blades of the Quori”
Kahiko1	all Olman exotic weapons	Dragon vol. 352
Pole Fighter	chosen “pole arm”	Dragon Compendium
Pole Master	chosen “hafted reach weapon”	Secrets of Sarlona
Serpent Strike	Longspear	Eberron Campaign Setting
Unorthodox Flurry2	chosen light weapon	Dragon Compendium
Whirling Steel Strike	Longsword	*Eberron Campaign Setting

The big problem here, though, is that every one of these feats requires the flurry of blows class feature—and you’ve given it up in order to get decisive strike. Nothing in Player’s Handbook II says that you continue to qualify as having flurry of blows—though it should, and really it only seems reasonable to me that your DM should let you. But RAW, you cannot. That requires some thought—see this question for more details.

Weapon Schools

Dragon vol. 330 also has alternate “weapon schools” for monks trained with different sets of weapons from the default set. Again, no dice on a katana.

Weapon School	Special Monk Weapons	Source
Okinawa (default)	Kama, nunchaku, quarterstaff, sai, shuriken, siangham	Player’s Handbook
Ghurka	Club, kukri, short sword	Dragon vol. 330
Arnis, Kali, & Escrima	Club, dagger, quarterstaff, short sword	Dragon vol. 330
Krabi-Krabong	Club, quarterstaff, short spear, short sword, spear	Dragon vol. 330

^{(I have my doubts about the default set having much to do with Okinawa—particularly seeing as the siangham is an entirely-fictional weapon—but this is what the article claims.)}

Fighting Styles

Dragon vol. 346 is another one with some options, but still no katana. Extending the Unearthed Arcana concept of variant monk fighting styles, we get the “Sacred Path of Heironeous” and “Sacred Path of Hextor,” (yes, “Sacred” “Hextor”—I didn’t write it), which allow for flurry of blows with a longsword or flail respectively, provided you have 4 ranks in Diplomacy or Intimidate (respectively) and the appropriate proficiency by the time you hit monk 6th. The real problem, of course, is that you have to hit monk 6th... though the way each style takes over your bonus feats and fills them with garbage (Endurance, Negotiator/Persuasive, Weapon Specialization) might be nearly as bad.

Oriental Adventures variant¹

And then, as HeyICanChan again points out, there’s Oriental Adventures, which says

Monks can use any of the following weapons with their unarmed base attack bonus: butterfly sword, club, jitte, kama, lajatang, nekode, nunchaku, quarterstaff, sai, three-section staff, and tonfa.

^{(Oriental Adventures, pg. 19)}

Oriental Adventures is a 3e book, and “attack with your unarmed base attack bonus” was the 3e equivalent of “special monk weapon.” Dragon vol. 318 is supposed to provide the 3.5e update for everything from Oriental Adventures, but it ignores the monk, so YMMV. Officially, this is legit, at least when playing in Rokugan, not that it helps you with your katana.

Aptitude²

Finally, as Miniman points out, a +1 aptitude katana would probably work with any of these feats, because aptitude is weird. YMMV hard on whether the DM is going to accept that, not that a feat and a +1-equivalent enhancement seems worth it to me. You can find aptitude in Tome of Battle.

Conclusion

As far as I can find, these are the only options. I’d certainly chalk it up to an oversight/monks never getting enough love, but you’d have to talk to your DM about that. Searching for this, I find numerous monk revamps that give the monk katana proficiency and use as a special monk weapon for free. For that matter, I’ve played in many games that ignored the monk special weapon restriction outright, and it’s never been a problem.

1. _{Thanks to HeyICanChan for pointing out Kahiko, Flowing Blade (and Reshape Mind Blade), and the Oriental Adventures monk rules.}

2. _{Thanks to Miniman for pointing out Unorthodox Flurry and aptitude.}