Nice TF2 reference.

But I think they hold to much pride in what they are to hide themselves.
yeah, i'd think so too, maybe if you want to meet the leader of their legion, they'd have a safehouse right under the nose of their most hated foe, and you have sneak through the strip like Dmitri Petrenko and Sergeant Reznov like in Vendetta in Call of Duty: World At War. (or like the All Ghilied up in COD4) another thing they can do is tell you to meet a spy in that building, and supply you with a uniform, but if you use your stats right on one of the other legion troopers, he will tell you thats the "joke uniform" that they supply to initates that dont get the Sergeants approval, the NCR will recognise you as a Caesar's Legion trooper with that fake unifrom on, and the sergeant gives you a real uniform as an apology. now that wouldnt be bad...or you could stealthily kill an NCR soldier nearby and steal his uniform. but heres something else to think about:
a soldier sticks out if he isnt in his uniform, so what if when you kill a guard and steal his uniform, a sergeant nearby asks you why theres blood on your uniform, or why its in bad condition, or ask why your uniform is in such better condition than everyone elses, he may sniff you out as an imposter through speech, or you can persuade him, bribe him, or do other options.
this could go for any faction really...