Here's a cleaner version of the support for quiet properties (the
downstream issue).

Example usage in schema.py:
nosy_count = Number()
nosy_count.quiet = True
issue = IssueClass(db, "issue",
                   type=Link('issue_type'),
                   [...]
                   nosy_count=nosy_count)


If there is still interest in an option to mute/omit selected properties
when calling roundupdb.IssueClass.nosymessage, I can update that part too.