arachnid: I've been having the same problem with finding decent bow wood for a self bow (I've got plenty of access to good hardwood lumber, but a lot of it has been in small stock so I haven't found anything but marginal pieces that I'm too afraid to use as a back). I've never worked with Ipe, but from what I've heard it's weak in tension comparatively speaking to its compression, so slapping a back on it probably would be the safe bet. I've been using strips of "decorative" bamboo (kind of a pain to process, and hard to find in the right diameter), paper bags, rawhide from dog bones (again, pain to process and inconsistent) and reinforced packing tape to back bias-ringed lumber bows for a couple of months now and haven't had any major problems. If anything, a cable backing should be serviceable and look pretty awesome.
I'd bet if you over built it a little you should be able to get away with 40#@26"...