As far as I know, the air intake on most powerheads is designed to sit above the surface of the water, and actually sucks air in. This means if you directly connected DIY co2 to this intake, it would likely suck out your co2 bottle and proceed to spray the yeasty sugary muck into your tank.
I do that with my pressurized set-up and its fine. However, two things to consider.
First, if you do not have a bubble counter/gas separator between the bottle and the powerhead it will suck any yeast mixture that gets into the line right into the tank. Depending on how much it could kill everything in the tank.
Secondly, if the air inlet is after the impeller it really wont do that good of a job of dissolving the CO2. I tried it with my AC20 powerhead and all it did was burp out big bubbles. Try sticking the line into the intake of the powerhead so that a mist is created by the impeller.
I highly doubt the pump will create enough suction to extract the yeast mixture (C20 FTW!). It is possible, and you'll know within a few hours of trying.
Back when I was doing the DIY yeast CO2, I ran the airline tubing into the intake of my XP2 filter. That worked pretty well, well enough for a lower light tank.
If you do feed the CO2 into the pump, you do want to make sure the impeller smashes the bubbles up, so introducing the gas before the impeller in either an intake strainer or other orifice is best. I have used a needle wheel type pump, made by Gen-X (the 1000 l.p.h. model), and had very good results with it. There is also some good information around on modifying the impeller of a regular powerhead to break up the bubbles more efficiently.
Well, you can, but most of the bubbles probably won't go into the powerhead. If there is some kind of intake strainer on the powerhead, stick the tubing into that in such a way that the bubbles will go up (bubbles rise!) into the intake of the pump. This will be a very effective way of adding CO2 to your tank.