Here is how I tested my change:
1. Installed
2. Verified that the acs-subsite master was dotlrn-master-custom
3. Changed it to dotlrn-master (which works fine for the default Sloan theme)
4. Created a community
5. Verified that the acs-subsite master was still dotlrn-master
So that should do it, right?
I want to release 2.2.1 alpha on Friday so speak up now if I have not fully understood the problem.