I tried something like that late last year that I picked up again sometime this spring (flad may remember). I dropped it because of memory issues with the pathfinding.
It rendered the polygons by using a mask. First, I filled a buffer with the texture. Then, I filled a mask-buffer with an arbitrary place-holder color. Then, I used allegro's polygon drawing routines to draw my polygon onto the mask using the whatever magic invisible color (pink/black/whateva'). Then, I drew the mask over the texture buffer. This left the polygon, surrounded by the place holder color. Fill all the placeholder color in with the magic invisible color and wa-la! A rendered polygon. I thought it was clever at least .
If you're interested, check it out! Complete with editor and everything.
and a readme
(i'm particularly proud of the scripting engine )
If you have any questions I'd be happy to answer them.
Just a note, theres a small memory leak from the pathfinder.
oh, and credit goes to jim sinha for the view-shrowding. turn it on and off with F1
edit: and yea, this is an abandoned project, I know the code is kinda hard to follow. And I know its not totally optimized, or the best way to do this.
edit2: and i may get a delayed start on the xmas hack, was up pretty late working on this post