Hi,
Rather than trying to get a D-Star radio to QSY in less than 5 kHz steps, you could try freeDV with your existing SSB transceivers?
D-Star occupies 6.25 kHz bandwidth on the transponder, but freeDV only needs 1.25 kHz (half of a normal SSB voice emission).
You can get freeDV (experimental) software here: http://freedv.org/tiki-index.php
Unlike the early days of SSB (which has been the subject of various patents) AMBE is unlikely to ever be free, even after the patents expire as DVSI Inc. treats AMBE as a TRADE SECRET, and the only way to get the codec is to buy the chip (a locked TI DSP pre-loaded with AMBE). DVSI's patents cover some of the technology used in AMBE, but these patents do not explain enough about the AMBE process or protocol for someone to write their own AMBE-compatible codec.
Once the patents expire, someone with the right resources might be able to hardware reverse engineer the firmware in the AMBE chip (very difficult!) and write a specification of the AMBE process/protocol, which someone else who has never seen inside the AMBE firmware can then use to write a "clean room" AMBE compatible codec. Depending on the laws in force at the time, it might also be illegal to de-bond the AMBE chip and reverse engineer the codec therein. However, all this is probably requires far more effort than it is worth - better to spend time, energy and money on developing an alternate codec that is truly "open".
FreeDV uses an OFDM modem (16 QPSK carriers + 1 BPSK pilot), designed to have strong resistance to the multi-path propagation typical on the HF bands that it was originally intended to be used on. However, this means that is has a ~ 12dB peak to average power ratio, which imposes linearity requirements on the transmit signal path. For satellite operation, where path loss is a much greater concern than multi-path, the use of a serial tone modem (single carrier QPSK, BPSK, GMSK, etc) for freeDV would reduce the linearity requirements of the transmit signal path making it easier for an otherwise marginal ground station to produce enough EIRP to work the bird.
73's ZL2WRW Ross Whenmouth