Hmm. Did you have the story mapped out or anything? I mean, was the rest of the fic just going to be grand ol' sex, or was there a punch-line or a narrative that needed to be resolved? Because maybe you could "sandwhich" the porn, write the beginning and the end and figure out how rimming got you to that end point.

If your mind just will not let you wander towards rimming, I'd suggest suggest working on other fic, reading other fic or rewatching episodes--you never know how inspiration will hit.

I have no idea if that was helpful or not. If you'd like a beta, I'll point out where it feels awkward.