jnisbet: The remaster supports the original game's way of loading fan levels via the command line. Simply launch the game with the -u command line parameter, no space, then the name of the .gob file, with extension. For example, if you downloaded The Great Escape (greatesc.gob), add the following: -ugreatesc.gob
In the game, choose Select Mission and then whichever mission is listed. Its name and the following briefing will be the same as one of the ones in the original game (usually Secret Base), but the actual mission will be the one that you loaded, assuming that you copied the file to the right place and got the command line correct.
Many missions do come with their own briefings in the form of a separate .lfd file, but getting them to work with the remaster is a hassle. First, you have to disable Enhanced Sprites to force the game to use the original dfbrief.lfd instead of the new hi-res briefing. Then, you have to rename lfd\dfbrief.lfd to something else and rename the custom .lfd file to dfbrief.lfd in its place... then, after you're done playing the mission, you have to rename them back... then do the whole thing over again for the next mission. Back in the day, that was usually automated with .bat files, so that's why the downloaded zips tend to include one, but they're not going to work with this remaster unless you modify each one.
Good information to know.
Seems to me that this is something a launcher could do automatically. Back in the "good old days" there were mission-launcher utilities for a lot of games. The famous "Dark Forces Front End" program did this all "automagically" for the original version, and it sounds like this would be fairly easy to create for the revised version as well. With the exception of automatically turning on and off the "briefing" issue you mention above in order to get the fan-mission briefings. I'm not sure how you could make that happen without exiting the program.
Of course, if we're willing to surrender the "enhanced sprites," it's pretty straightforward, but if you want those in the game, it sounds like you'd have to give up the fan briefings. Which sucks.