You could always post trade requests to the X/Y boards (at least, until they set up a dedicated trade board), for the Pokemon you're looking for. That would be easier than trying to get someone else's save file to transfer to the Bank (do you have the game to which the save file corresponds?).
I've got Y and am thinking of picking up Black 2 so I can have a nice team for some online battling, and I could beat it and catch whatever pokemon way before 12/27 but downloading a save file would be much easier
Not if you'd have to jump through who knows how many hoops just to be able to do that. In theory, since there's a device out there (I haven't seen it, but others on the forums have mentioned it) that's reportedly able to let you copy save files from/write save files to a DS game, you could write that save file you downloaded to the game it belongs to. The problem is, I don't know if Game Freak/Nintendo would find it suspicious if, say, for example, the person who uploaded the save also uses the Bank... then two people would be trying to upload the exact same Pokemon from the exact same trainer. We don't know what the Bank looks for in 'hacked' Pokemon, either, so for all I know, there's no guarantee they might somehow see the cartridge has been 'altered' (by uploading a save file to it), and deny the whole thing, or that some or all of the Pokemon on that save would be considered hacks by the Bank.
Also, as I don't know what the device is, I couldn't tell you whether or not they even work with the 5th-generation Pokemon games in the first place, or if the saves GameFAQs hosts are compatible with it. (OR where to get it. :|)
It's still probably much easier to just go on the X/Y trade boards and ask for some or all of the Pokemon you want to get (and if they'll trade you those Pokemon for 'junk' Pokemon, like you can do on the B/W/B2/W2 trade boards.)