...that sounds like a great idea, although the only downside I could see would be a lack of fittings, where the PVC or ABS pipe has elbows, 'Y's, endcaps, reducers, etc.

Just make sure that there is no chance for stagnant water in there, and you should be fine. For a pipe holding six plants, I'd put a spray emitter in between 'em all, and ones on the outside, for a total of seven emitters per six plants.

I remember when my friend in Calgary cropped out, and it was amazing how long the roots had gotten in the pipes he was using. His plants were killa.

As to what pump to use, I don't have any input fer ya, but perhaps some of our other denizens might have some advice...

Happy Gardening!