Even when the clients selects are sent organically (and not generated by my web gallery) they're typically in the form of a list that can be copied and pasted.
So simple. Thanks for the inspiration to look again.
Simultaneous searching for multiple files just stopped working for me. After a few hours of tinkering and 2 hours of almost useless support from Adobe, I figured out that during a search for multiple files none of the files can have a space in the filename. If just one file has a space in it's name LR will not find any files from the search.
It's clearly a bug because LR will find a single image with a space in it's name.
I've asked Adobe Support to fix the bug in the next update and they've assured me they will; we'll see.