Ok, this actually makes a lot of sense and seems pretty obvious, but for some reason I didn't think about it until today...
With the release of 1.75 I've been in love with the 512x512 terrain textures, so much nicer to look at. But, they come at a heavy price, at 171kb per texture any map you make with it will have a huge PBO file (think way bigger than 20mb). That will mean that anyone running less than 512MB ram probably won't even touch your map since they would have to remove most, if not all of their other addons.
So, I figured, lets try a pbo file with the exact same name full of 128x128 textures with the exact same names on a map designed using the 512x512 textures. Well, it works
So, if you, the mappers, pack your terrain textures outside your map .pbo you can have two seperate sets of textures for people to download. Also, at 1/10th the size people can download the lo-res version first to see if they even want the map in the first place.
The only thing I haven't been able to test is running the two seperate version over a network (come to think of it, I've never even run OFP over a network at all). That may be the only real catch, even then it would be a simple matter of including the lo-res version with the hi-res version and a simple bat file to swap them out, then make sure the servers/hosts run the lo-res version.
Now, if we had external file access from within OFP we could use this same theory to apply seasonal changes to maps. ;D
So, I'm off to do lo-res version of mi hi-res texture packs, have fun...