Quick update - no luck on using the BK2421 so far!
Moved on to trying a different transceiver, I had an nRF24L01 lying around so thought I'd give it a crack with this code, to no avail - drone still does not pair.
Took delivery of another two UDI U818As today and will proceed to logic sniff them with the BusPirate and see what I'm doing now. The guys over at Deviation list them as 'unsupported' however, which is slightly terrifying.
For the moment, however, I'm going to start developing my FPGA+MCU board and leave the transceiver/timevampire as an empty slot to connect the SPI pins to, there's a good chance I might have to change drone or transceiver model suddenly so I think it's best I make that an easy transition.
Board designs soon.