Currently, it's possible to host a Workshop maps rotation by ignoring the mapgroups and using mapcycle.txt. As long as you have a Steam API defined per the guidelines, you can manually add Workshop maps to the rotation like so:
Code:
de_blahblah
cs_blahblah
workshop/12345/awp_blahblah
workshop/67890/fy_blahblah
But the problem is that the nextmap and randomcycle plugins are limited to strings of 32 and 64 bits in length. Adding on the paths generally puts them over this, and the cycle gets hung up on the same map quite often. It's easy enough to recompile the plugins after changing the string lengths to something larger like 128 or 256. I'd just like to suggest they be made longer by default for future releases.