but that would add the additional question: how does one tell in what one is specialized? Can everybody enter his or her own specialization (how believable would that be and where is the limit)?
I know a forum where you could submit your choice of subtext (the location you see "member" or "on probation") to the staff, which would in their own time fill in the rank in question. It would be a bit of administration to manage the initial flood of requests, but if you put the limit above say, three stars and an established reputation, I'd not expect the overhead to get above one rank change every two weeks.
Alternative option is to have a freetext specialization field in the profile, which need not show under each post, while it can on its user page, much like the occupation field. Essentially, you can add some useful information without the need of spamming it. I also do not think that requires any censoring - if you suck, your specialized subject will also suck, only not as bad as the rest. The other half of information can be gotten from the post counter
I don't see any serious issues with the second alternative - if a cheap solution is needed, you can just reuse the occupation field.