[Mechvet]

R30 (as of 2016) allows modifying a motor housing for the purposes of mounting, which certainly opens the door for structural mounting of a CIM.

It may be possible to entirely replace the output shaft plate of the CIM with your main anchoring plate. Only restrictions I'm seeing from the rulebook would be ensuring that your design is not lighter than the original, and that the electrical and mechanical operation of the motor have not been modified.