About the Voigt pipe, I've noticed some small drivers lately that would probably be good for projects like that. Eminence is making their new 6" array drivers that might work well, and Parts Express is always running specials on Tang Band, Eminence and Vifa drivers that should be well suited. And of course, there is always Fostex, which really aren't too expensive.

The Goldwood you've mentioned looks to me like a potential canidate too. I would expect it to work well in large ported cabinets and open baffles. Maybe Martin would have some insight on what he thinks about using it with a transmission line or tapered pipe. The price is certainly right!