Can you possess a creature, using the magic jar spell, if you are in an extradimensional space (assuming you can see the humanoid) in 5e?

Question

How does the spell magic jar work with an extradimensional space such as a bag of holding, handy haversack, portable hole, or the Genie Patron's Bottled Respite feature?

Assuming you are able to see out of it are you able to attempt to possess a creature? Is there anything that allows/forbids this RAW or is it up to DM interpretation? I'm asking because if you are, I think it would be a really cool thing to make your character effectively a cursed magic item.

Side note, are you able to partially have something inside one of the listed things? They don't magically teleport inside (with the exception of bottled respite), you put something in there and take it out; it transitions from the plane you are on to the extradimensional space. This is important as you need to see a creature to possess it.

Answer 1

Depends on the space

Each extradimensional space comes with their own rules, there is no unified set. The consensus is that you can cast spells out of a portable hole. To contrast, with a rope trick, you cannot cast any spells accross the entrance:

Attacks and spells can't cross through the entrance into or out of the extradimensional space

As this is not as clearly called out in all cases, those will need to be decided by your DM. The bottled respite vessel in particular does not say anything about an entrance, so it is unlikely you would be able to target anything from within, as you have no line of effect, and cannot see the target.

For your side question: again this depends on the space and in the end will be up to your DM to rule on. With rope trick, or portable hole, nothing forbids "leaning out", even though the movement rules if you use a grid do not really have a concept for this: you are in a given space, or you are not. With other spaces, like bag of holding or handy haversack, you cannot actively enter or leave, you can just be retrieved and it is described how that works in the item (note that you will suffocate and die as a breathing creature, if you stay in there longer than 10 minutes).

Practical issues with this idea

While possessing a body, you can use your action to return from the host body to the container if it is within 100 feet of you, returning the host creature's soul to its body. If the host body dies while you're in it, the creature dies, and (...) you return to the container if it is within 100 feet of you. Otherwise, you die.

The distance to an extradimensional space is undefined. In some cases like with an open portable hole, the DM may allow it to be normal distance. In most cases, it will be considered as infinite.

When the body you inhabit dies while the receptable is in the extradimensional space, you also will die, as the container is not within 100 feet of you.

Answer 2

Up to DM interpretation

You are definitely going to have to work this out with your DM.

The magic jar spell says:

You can attempt to possess any humanoid within 100 feet of you that you can see

So, can you see it? Maybe you can come up with a way to see a creature outside of your extradimensional space.

Are you within a 100 feet? Debatable. It an "extra-dimensional" space, as such, distance is hard to determine and is going to have to be determined on a case-by-case basis.

Work with your DM

The things you're describing aren't supported directly in the rules. Depending on the specifics, they might be interesting and fun, or they might not really fit in with the game.

Also, how things transition into the bags, haversacks, and holes aren't defined in detail.

You're already a cursed magic item

If you're using magic jar, you're already a cursed magic item. It's not going to really matter if your jar is inside something else.

It's not clear what element of surprise you're gaining by being in an extra-dimensional space.

You're probably already hiding in plain sight. The jar can be a decorative bottle on a shelf, a fancy box in a closet, or a diamond in the crown of a queen.

For that matter, every gem of any significant value you find in any treasure chest or monster's hoard could be a magic jar. Worth thinking about, isn't it?