Not that I'd even consider them for a substantial wash job, but any harm in using a wet paper towel to remove some bird poop?

 

"Paper" (or cellulose) is one of the most abrasive substances on Earth. You can cut almost anything with scissors and they will not dull for a very long time. Start cutting paper with scissors and you'll dull them to the point that they won't cut anything else in very short time. Use it on your paint with extreme caution.
I use paper towels and Windex to clean off bug splats after a long hiway drive, but that's only on the part of the nose that has 3M film on it.
Better off to get a microfiber cloth as your "poop cloth".

 

Good to know, thanks!

Now, since I have done this in the past, how concerned should I be, and what can I do about it?

 

If you had a good wax or polish on the surface, probably that took any abrasions that may have happened, so no worries. If not, then give the area a light work over with scratch and swirl remover (heck, do the whole car - won't hurt) and re-wax.
When you say "wet" paper towel, there are degrees of wet. If it was just moist, then there would be more risk. If it was dripping wet, then less chance of any harm as the water would act as a lubricant and as a diluter of the bird goo. The bird poop probably has just as much abrasive substances as the paper towel. It's just one of those lovely things that we, as owners of fine automobiles must cope with daily. Can't always get around some of the bad things.

 

it was explained to me many years ago that you wouldnt use wood to rub on your car, and paper is made from trees-
that kinda stuck with me and helps me remember not to use paper towels on the car...
a swirl remover or paint cleaner should repair any paper damage-

 

I don't know about this... I've never seen an LSP that would be hard enough or thick enough to absorb the kind of marring you get even from washing and drying, much less from using paper towels.

If it's just a small spot or area, some Meguiar's Scratch-X should work well by hand to clean up light scratches.

 

Paper towels would definitely leave swirl marks in your paint. I always carry a couple of MF towels and some QD spray in my trunk for bird crap occasions