I've thought about some icons like a switched off lighbulb or a thumbtack, but they seem a bit crappy. I'd use a textual link or a checkbox.
Btw, are you gonna avoid people to half-stick several posts in one thread?
There could be several interesting posts in the announcement by of the public beta worth the half-sticking.
Given this case: what would happen when you press the lightbulb? Could you make the digest-page restrictable to fid (and tid)?
Bit more complicated, though, but not too much
ahm... what if you put a link to the interesting post as a small message at the top of the threadspage (like the report-box)?