They would need it to be a relatively new IP but of high quality, or relatively old and definitely high quality to ensure sales. Something people may not have checked out but now can at a fraction of the price. That is part of the draw to the indie bundles after all; that you get games you may never have thought of before.
THQ certainly has a plethora of games it can dip into as bonuses, not to mention the RFA DLC and all the SR3 DLC.
But since a lot of people already own many of their games, they're no surefire way to "gain" more sales. If they were smart, they'd make the BTA bonus beta access to CoH 2 and/or SP: Stick of Truth (probably easier with CoH since it has MP). They'd get a hell of a lot of buys and it would incur no additional expense for them. Win-win.
Putting betas in there would definitely be possible. If Valve allowed them to upload the content to Steam, so keys could be issued, then people could get early access that way, and if the actual beta testers use the Steam client for testing (sometimes QA doesn't and instead uses their own channels, but then they can miss things from a bad cook) they can use the password protect beta channel. It would likely be worth it too, if they are confident in the game. Get it out there to get enough community interest that at least the one project is something people will want. Drive interest to drive investment. Of course there would then be the risk of ruining interest if the beta isn't in a solid, playable form.
We'll see, won't we.