There's a sentence taken from Hewing's book (the writer of Advanced Grammar in Use):
Write v if the italicized parts are correct. If they are wrong, correct them.
We are meeting at seven in the Globe coffee bar. Can you be there, too?
The key answer says it's correct, but it also says that were meeting is also possible, although there's no further explanation.
Also here is related discussion about this problem, but it still hasn't answered my question whether it has something to do with politeness or an event involving the past. However I can't see the past sense in that sentence above. If it's for politeness, then why can you... instead of could you...?