Although it seems there’s a branch of RFM12B-Linux which can do OOK already. I’m not entirely sure how this is going to work out, but I’ll keep posting.
I found some problems with RFM12B Linux, specifically it doesn’t work at all on the Raspberry Pi B+ 2. I’ve created an issue for this but I might be able to fix it if I can get some tinkering time. In the mean time I’m testing things with the original model B.