My wife plans on tanning topless on the main beach. There are mixed reviews. Some people have said that it is allowed, while others say that it isn't addressed in the rules so it isn't allowed. I'm sure that most people will be discreet about it and everyone is on vacation and won't mind anyways. I say go for it.
The rules state that there isn't anywhere for topless sunbathing at CSS, but we've seen it on the main beach. If nobody complains, you might be ok, as long as you don't draw attention to yourself.
It also might, and I repeat, MIGHT be acceptable if you are already known by the group at SSB during you stay and you just happen to need to cover up for a day. You will most likely feel weird, since more people will look at you since you'll stick out. I painted a bikini on my wife last year and she said she felt really self-conscious because people were looking/gawking at her to see what was going on.