This would be my $0.02 solution. On a weekly basis, when shocking with dichlor, I would remove it and dip it in the superchlorinated water upside down. Let is stay there for a minute or so and let the chlorine kill the buggers. If you're doing it on a weekly basis, the buildup shouldn't be to the yucky stage. I tend to do that with my filter cover every shock just to keep it from getting bad.
That would be a great idea, and i considered it, except for 2 things:
1 I generally shock with MPS, not dichlor
2 The stuff sort of sticks in there, so just swirling chlorinated water in there wont necessarily get rid of the adhesed goo.
I think my plan for now unless something better presents itself is to wrap a pencil in a small rag with bleach water up in there once weekly.
Yesterday morning I checked to see what it was looking like- there is still a fine irregular layer of tan/grey film, but it has no odor this time. Perhaps is it more "soap scum"ish than anything. I have no other build up of stuff anywhere in the tub itself though.