Hey thanks for the reply DrO.
I had actually just installed Winamp 5.666 not too long after I posted this, and it wouldn't take a mapped drive as a Send To target there either.
I think I might have been using an old third party plugin that's long gone...
A few minutes ago I tried one of the USB3 ports right on my mobo instead of my case's (apparently janky) front panel and was able to get a much more stable connection there... Windows Explorer can actually access the phone's storage without hanging now.
So I gave it another try in WACUP and the pmp_mtp works great!
It's actually pretty slick.
Sorry to waste your time here but I'm happy to report that WACUP's MTP support works fine for me at least, using a Samsung A53, as long as the USB connection is good.
Was totally barking up the wrong tree with the mapped drive thing, MTP is the only way to go for linking WACUP to Android apparently.
Thanks again!