Yes, I have been wrestling with this also for the same reasons you all have exposed above; shakes when trying to do something small and detailed at the end of a long (& often tiring) painting period, not detracting from the scene, etc. The fine point Sharpie idea (see that Forum thread) sounds like what I have been looking for.
I'm thinking on the basis that many landscapes could have a small old white milestone marker beside a track/path, white rock, signpost etc that could fit into the bottom somewhere. Once the painting is dry, my hand/arm is more rested and I can do it on a horizontal surface, I could print later with Sharpie; all of which should help me achieve better control. I'll probably just use initials and year.
Aim would be that it could look to be a valid part of the scene. As suggested, putting any other details on the back of the frame is a good option. Any thoughts?