To the best of my knowledge the maximum baud rate is 115200.Please check Rabbit 6000 processor reference manual at digi website.
On GitHub, I recently added an HDLC update for Dynamic C 10.72 that improves HDLC support on the Rabbit 6000 processor, and adds an HDLC sample program. You might want to take a look at it.
The README for that repository includes details on how to use Git to integrate updates into your installed copy of Dynamic C 10.72. Digi plans to release a “10.72A” version by the end of March that will include those HDLC changes.