I'd like players to be able to generate XP bottles, to that end, it'd be nice if they could click on an enchanting table and "fill" bottles that way from their own XP bar (at a loss, of course).

It could also be useful for 'vampire' effects that drain XP, perhaps on receiving damage or hitting a mob.

This sounds like something that's likely already possible to set up... unless I misinterpreted your meaning.

I thought so, too, messed with variations using negative numbers and whatnot. Zarius said on IRC that it isn't. So, feature request. We'll see if it goes anywehre.

Well, the part about right-clicking an enchanting table with an empty bottle and getting back an XP bottle should be possible, at least. That just leaves draining the XP... which I guess is what this is really about.

yep. Except click, not rightclick. rightclick activates the table.

Nevertheless, it should be possible.