Apparently I hardwired the bitrate in the RFM69 library, so you need to set the RFM12B bitrate as follows:
radio.Initialize(NODEID, RF12_433MHZ, NETWORKID, 0, 0x7F);
You also need to disable encryption on both ends (they use different algorithms and I've not rewritten the RFM12B version because I don't use encryption at home). They should then talk to each other (currently testing the RFM69 running Struct_send and RFM12B running Receive). Not very intelligible on the console ouput, but should demonstrate them working okay.
Note: I updated my RFM12B fork on GitHut to set the default bitrate to be the same for both RFM12B and RFM69, and I now print a console message if you try to use encryption on the RFM12B.