It actually
is legal to produce and sell fanwork in Japan, hence the popularity of doujinshi.

They sort of let it slide here when the doujinshi get imported, and like Snowdrop said, at conventions pretty much anything goes. If you were to produce a doujinshi in the US and distribute it widely it could get complicated, though most people understand that selling fanwork is a labor of love, not profit.
Unfortunately, at anime boston last year all I found were Sess/Rin doujinshi, not Sess/Kag.

I'll be on the hunt again this year!