I think I'd have to agree with bigben. Adding air to the system would complicate matters. You'd have to include a water trap to get the air back out of the system before it returns to the pump.
Well, on second thought, I guess the reservoir could act like a trap. Maybe there'd be no need for additional equipment other than a pressure relief valve on the reservoir to let the air out as fast as you are injecting it in. Another thing to consider is that by constantly injecting air, you'd open yourself up to contaminating the water.
I still like that piston pump idea way back there. I've been looking for something in the 2 gpm range that would be small, quiet, CHEAP, and run on 110VAC or 12VDC, but I haven't had any luck.
Speaking of luck, I had lightning hit my house a couple of nights ago and fry two of my computers' mobos (possibly CPU's too, but I don't have anymore mobos to troubleshoot with now).
Coincidently, the two computers that fried both had expensive (to me anyway) APC surge protectors on them. The third computer (a POS Dell) that had a cheapo surge protector is fine.
So, looks like this little waterblock project may get put on the back burner until I get those fixed.
PS - I don't think I'd recommend APC surge protectors to anyone.