I posted this message at the emactulation.com forum about this issue:
It turns out that this is a problem with any application running under SheepShaver that tries to copy "styled text" to the clipboard. If you format some text in ClarisWorks, for example, and copy it to the clipboard, you get an empty clipboard.
So it's a SheepShaver but that will presumably never be fixed, alas.
There's a patch here for copying styled text to the HOST clipboard from SheepShaver/BasiliskII under BEOS, but it doesn't seem to apply to this issue.
EDIT: It turns out that the relevant-looking bits of that patch are already in the code that I used to build SheepShaver, and presumably in other current builds also.