Any other examples of using :mrf_subchain to do custom processing?

Just wanted to see if anyone has other examples for :mrf_subchain. I was wondering if it might be possible to leverage this to set up a filter for specific users so that the instance can reject DMs that mention the user in question? I know that there is the module to block non-public, but that’s for all users on the instance. I was wondering if there might be a way to set up a subchain to accomplish this for specific user(s) who might need it/request it.

There are so many experienced people here and just wanted to try and see if anyone had done things with this.

Thank you.