@CraftyCritter Hey I didn't see the forum post, I was linked the video directly but I did leave a comment on the video.
Anyway the reason you couldn't load it without a patch was because we only enabled unsigned fastfile loading for "mod.ff" which was a long time ago when primitive mod support was first added and no custom map tools existed at the time.
But it should be fixed and on our staging branch soon to allow loading any unsigned fastfiles when an fs_game is set.
If you'd like you could send us an example custom map fastfile to test the usermaps functionality that we had also added a few months ago.
Feel free to report any other bugs you encounter either here or send a friend request to me on Discord account name jezuzlizard.