Fixed in r740864.
There is a new field for the scm comment prefix on the 'release prepare' page, and the value is passed through to the release process.
If no value is provided, it will use the default of "[maven-release-manager] ".
A possible enhancement would be to append a trailing space if none is present.
This could use an automated test, however the current tests look for contents in files, and I don't see how to test that the commit had the correct comment. As long as we're passing the value through correctly, the code doing the actual work is over in Maven [Release|Shared|SCM].