I cannot get HDLC working on my RCM6700.

We have a patch for the HDLC_packet.lib is incorporated into our latest release Dynamic C 10.72E.  This patch adds support for Rabbit 4000 to Rabbit 6000 boards. 
Click on link;  https://hub.digi.com/support/products/dynamic-c/

If you would like to add the patch to Dynamic C 10.72 instead of using the latest release, you can find it on Github;
https://hub.digi.com/support/products/dynamic-c/?path=/support/asset/dcrabbit_10-on-github/