I think I read somewhere that you can mention the name of the mixer, but you cannot include a link, just mention the name.
That might work sometimes, other times it won't. Theymos has applied word filters to known associations with mixing services. But I don't know if those filters apply only to URLs or any mention of such mixers and similar combinations. Anyways, if you mentioned a banned word, the forum's settings will change it to show "
banned service" or something along those lines.