It's allowed to put referral link on OP if the topic isn't a spam and if it contains a some good information like a long detail thread. BTCGOSU topic can be an example in that case.
There's a statement from theymos on this if I can remember correctly, I can't find it right now.
Correct since it’s not spamming. I believe it’s acceptable to post referral code here that is part of your service offered in the forum.
The rules of spamming referral link was introduced when ponzi scheme is very popular on Bitcoin that makes many user spam referral code on random thread that is not related to the topic. But posting a referral code for your own service in the service ANN will probably accepted by theymos.