I was pleased to discover that someone else has made a Fallout 3 mod to swap out the most egregiously revealing (female raider) armor. Here is that mod!
Note: this mod does not alter some of the other immodest clothings, and also does not change the fact that when you steal someone's armor, they have only got underwear beneath. I may attempt a more comprehensive mod of my own in the future, but this will do for now. Thanks, internet!
Modesty Go! (Nexus Mods)