setFlag/clearFlag restricted to basic flags

When using setFlag/clearFlag the flags are restricted to some basic flags. I'm currently trying to set and remove flags like $NotJunk (set by AppleMail I think) or $label2 (set by Thunderbird as "Tag: work"). There are also use cases to flag them with other custom flags for spam learning, spam filtering, potentially dangerous or whatever.

Normally these custom flags SHOULD begin with a dollar.
There is a IRTF Internet Draft Proposal which lists some important flags:
http://www.melnikov.ca/mel/Drafts/draft-melnikov-imap-keywords-00.txturl

It would be nice to remove the restrictions, so the developer can use all kinds of flags. At the moment I have to overwrite the method to remove this restriction.