I've had some Green Arrow arrows in the works for a while as well. My idea was to use 1/2" CPVC tubing, connectors and caps to create some of the capsules. I can't remember the thread size, but a common one fits into the arrows I had picked up. A flanged T-nut at the back of the capsue added a nice look, IMO. In one of the JLA mini-series, a rocket arrow was shown, and one of Estes' small plastic ready-to-fly rockets would work well there. I'm sure a lot of that would work for Hawkeye as well. Also, I think there was a page on Hawkeye's arrows in the old Marvel Handbook Book of Weapons, IIRC.
Jim