OK, still get the same error referring to LE not being able to follow links to URLs with ports other than 80 or 443.
So I did the next logical thing, took the 10000 port off the RewriteRule. Now I get a slightly different error, but the attempt still fails.
Oh, and I did restart Apache in between each attempt.
If @Joe or @Ilia don’t chime in here tomorrow I’m going to report this as a bug in the Pro forum. Either my statement at the end of the OP here is correct, or I’m missing some information required to complete what should be a very straightforward operation.