So Lethamyrs website gave very concise instructions on how to get a workshop map split-screened via Custom Map Loader before booting the game up and then loading up Underpass once in-game (thank you very much Leth).

EXCEPT for Dropshot maps, they don't show up, and the only two things I found for a fix didn't help me out, I read to load up Dropshot in-game instead of Underpass and that just gets me the normal Core 707 map, and I also read that yer supposed to change the name of this file:

Labs_Underpass_P.upk

to this:

ShatterShot_P.upk

...and since I'm bothering to type all this out, you can guess how well that worked for me. None of the maps I've downloaded even have "Labs" or "Underpass" or "_P" anywhere in their filename, they're all unique names based on whatever the respective map actually is (i.e. ObstacleCourse1.udk or Minigolf.udk etc.). Although I did try to rename one to "ShatterShot_P.upk" anyway, just in case that ended up working, which of course it did not.

I'm obviously not too savvy with this stuff, I'm just a google troubleshooter who seems to have hit a proverbial wall and can't make some of his maps work...and I love me some Dropshot. I appreciate any and all help, thanks very much in advance!