I'm very puzzled about what to do here.
I was hoping to use escaping, but I'm not finding any reliable escaping rules in the MongoDB documentation.

In this section: http://docs.mongodb.org/manual/faq/developers/#faq-dollar-sign-escaping
The suggestion is to use the "Unicode full width equivalents" but that seems like a nightmare to debug for users!

Replacing "$" with "_" sounds tempting but it's ambiguous. Not that I expect people to actually use underscores in classnames which would otherwise result into an identical mapping, but still it's not bullet proof.