Another group has been considering something similar for 'employers' (agencies, brothels etc).
(For the rest of this post, group 'A' allows employer 'B' to display the good conduct logo.)
I can see the advantages for both sides. It builds awareness of A and allows B to have a competitive advantage against its rivals, both in recruitment terms and in getting them clients. It sets out to everyone, workers, employers and clients, that there are standards that everyone should uphold. It can be used as an argument with government etc that this would be easier if it weren't for the laws making it impossible to be a employer legally.
The 'buts' are that it'd ideally need to be done in such a way that B's site display the logo via a direct link to the actual image hosted on B's site (so if the 'award' is withdrawn for whatever reason, this can be done) but this would let A see how much traffic B's site gets. Not everyone may find this acceptable and it's also not really relevant for your scheme.
Deciding exactly what should be in the guidelines B would sign up to was also proving 'interesting'. I can remember one particular argument around OWO: while I'd expect to have it say something like 'no insisting on any particular service', there are people who think that it should insist on the highest levels of sexual health safety, and in particular no OWO.
Personally, I refuse to use condoms for blow jobs (I'm aware there are risks, but I find them acceptable and I love giving OWO too much) so it'd be unacceptable for me to have someone insist that I do so. Spreading it to male escorts generally would also be difficult - while there are plenty of female escorts who won't do OWO, the percentage is much, much lower for male escorts.
This brings up another issue: either way, any sensible set of guidelines is going to involve B effectively admitting that what they're up to involves prostitution, and again not everyone is going to be happy with this.