raft3plus and brendodendo, this is an awesome idea, and I'd love to see it happen! Please keep me posted. We've thought about product extensions which would zip on and off to make sides and a full tent. Hopefully we'll get there at some point in the future.
Unfortunately, I can't sell you the frame alone. It only comes in a kit from the manufacturer. I've looked around on the internet, and frame-only kits are just as expensive (if not more) than buying the whole bimini. That said, the top is a separate piece from the frame, so you could order the sharkskin fabric (least expensive) version and just not mount it on the frame. Keep it as a backup, or better yet, sew some zippers or put some buttons on it to attach your tent pieces. The fabric is waterproof (as is the Sunbrella which is heavy marine canvas). Hope this helps and good luck!