I was looking at this earlier and complaining to Klar how much it would suck to have to dump out of ArmA and reload a different shortcut when changing to a desert map, to load up your arid camo replacement units.
But I forgot that a lot of these unit packs add them as extra placeable (and playable) units - it's only the actual replacement packs that would change the default USMC uniforms.
The USMC infantry pack referenced above claims to add the extra placeable units as opposed to being a local-only replacement kit.
For vehicles, something like this would do the trick:
http://www.armaholic.com/page.php?id=8140. It just adds new classes of vehicles to the editor with arid schemes. That pack and the MARPAT pack Dec linked to are about 20mb a piece.
The downside is that players would need to have these arid camo units in order to play the missions, but the experience player-side would allow for seamless transitions between the forest maps like Chernarus, Utes, and Sbrodj, and arid maps like this one.