I think that 1Mbps is possible using the HDLC libraries. I don't know enough about the differences between SDLC and HDLC, or what data encoding you're using.
You might want to read through the HDLC section of the Rabbit 6000 Microprocessor Users Manual (http://ftp1.digi.com/support/documentation/90001108_H.pdf) or the Lib/Rabbit4000/Peripherals/HDLC_PACKET.LIB library.
If you have hardware, you could write a quick program to open one of the serial ports with your requested baud rate, and look at the return value of HDLCopenE() (for example) to see if it was able to configure the port for that rate. You could even add debug output to that function so it prints the value of "actual_baud" after calling SetSerialTATxRValues().