Yes you can copy the same save game to many USB keys. Usually both my friend and I will get a copy of our save game when we're finished gaming.
I'm sure there are games that refuse to allow you to copy save games. I believe (but am prepared to be shown wrong) that Rock Band saves can't be copied if you've downloaded DLC. But if you can copy it, you can copy it many times.
I'll give two examples, starting with Borderlands because you mentioned it. Regarding the types of games, we only play games that support local co-op.
Each of us can log on to our PSN accounts regardless of whose PlayStation we use. Borderlands saves are bound to PSN ids. So I'd log on to my account to make my character and he on his. Then, one two-player splitscreen session later, copy the saves onto separate USB keys. (mine on mine, his on his) (Incidentally, we learned this the hard way. Originally both of our characters were on his PSN id. It took me a while to 'liberate' my character and move it to a save game under my PSN id.)
The game we're currently playing is Sangoku Basara Samurai Heroes; the way we move saves for this game is the same for most other ones. The save is bound to a PSN id, so we just have to remember whose PSN id is the right one for the save game, and then just copy it to and from a USB stick.
Both me and a friend of mine have PS3s, and we take turns visiting each other's homes to play games. So we're in the target audience for this feature.
But we've been getting by just fine by using USB keys.
So
Build a system that even a fool can use and only a fool will want to use it.