Over the course of a few days, my Tour Easy recumbent developed a slight squeak that turned into a definite creak, then the seat started shifting slightly under hill-climbing forces. Of course, no force I could apply in the garage caused the slightest squeak / creak / motion. A decade ago this was due to a sheared screw at the dropout, but everything seemed to be in good order.
So I applied a drop of penetrating oil to each of the many joints in the seat hardware, went on a few more rides, and eventually the seat started moving with normal pedaling forces.
The left strut clamp looked fine:
OK, it looks grubby. I’d rather ride than lick my bike clean.
The right clamp definitely showed signs of motion:
I extracted the strut assembly, degreased the clamps, reinstalled in reverse order, replaced the nuts, snugged everything down, and it’s all good again:
Yeah, I should have replaced those screws, but I didn’t even have to take the wheel off, sooooo …
The Smell of Molten Projects in the Morning 
Good fix. Optional improvement: after degreasing apply a dab of coarse valve grinding compound, then reassemble. In this case friction is a Good Thing.
If it slips again, I’ll grit it in place: thanks for the suggestion!
Of course, a dab of grit from the chain would probably work as well, given how rarely chains get cleaned around here. [heavy sigh]