It's the Zope CMF property-sheet-counterpart to "snakes on a plane". Here's how to do it:
Why would anyone go through all that?
I had a brief e-mail chat with Martin Aspeli where it was established fairly well that certain control panel-like properties go well with the new registry for Plone and others don't go well at all, namely those that are more complex than numbers and strings. Rich text values are such complex properties.
The very same limitations apply for the venerable CMF property sheets. But on the other hand, you've got to put this stuff somewhere and hiding it away in some obscure annotation isn't exactly transparent.
So in a paean to the legacy software that the CMF is, I ended up storing the rich text value in said manner and it isn't all that awful.