Something like set service pppoe-server authentication radius-settings rate-limit vendor 'XXXXX’ would do the job.

I tested it adding the vendor to the pppoe.config and worked.

This is an export of my [shaper] section:

[shaper]
verbose=1
vendor=Mikrotik
attr=Mikrotik-Rate-Limit

Edit1: The Mikrotik Dictionary is installed.