I was looking at fully submerged power heads today, and I noticed they have intake hole for a air line. Would hooking a DIY co2 up to this air intake work? Or is it too much water movement to be effective?
Try this article on DIY CO2. Its the most complete rendition I've come across. Definitely worth a few read throughs for anyone interested in DIY CO2. (Be patient with the loading time, it seems a little sluggish).
Way down towards the bottom there is a description of a type of DIY reactor that uses a powerhead, although I'm not sure if its what you were visualizing. There are plans o'plenty for that particular gizmo, just search for "DIY CO2 reactor" or variations on that.