I always thought that having a 3 some with someone when your in a serious relationship is such a bad idea because you might ruin the relationship but with this setup you don't have to sweat that. You should work that angle and see if she is down. If nothing else you will scratch a hella fun item off the "you should do before you get married" list.
I actually had a friends with benifits type setup with this girl for about 8 months to a year. She was pretty and we had good chemistry but she just got out of a long relationship with this guy and I wasn't looking for a serious relationship at all. At that point I had never really dated yet and didn't want to. She actually told me that she was attracted to me because I was so anti-dating. Sometimes I would see her out and ask her if she wanted to hook up later and she would be like "hey the nights early, if you still want me to come when you get home call." It was fucking awsome, like get laid insurance. Could hang out and play the numbers all night and still got something to fall back on. After about a year or so she stopped answering the late night request and that was it but it was really cool while it lasted. We never actually hung out during the day light and she would always come over and then split right after we hooked up. Never stayed the night or anything. I don't think I ever saw her in the day light and we never did anything together, no movies, no watching TV, no going to eat, nothing but hooking up. The guy she was dating for years right before we started messing around was actually a childhood friend of mine who I ran into out at the bars when I went to college and he was always with this hot girl who was his high school sweet heart. I was always attracted to her but didn't want to cross that line. She used to come to party's at my house late when they were dating and want to hook up but I would say no and then not be able to sleep kind of thing. One night a friend of mine came by who knew what was up and told me that she was out looking for me and had broke up with her boyfriend. I went out and found her and then it was on for like a year or so.
I wish I had a disaster story for you but my freinds with benifits setup was fucking killer but I wouldn't have dated anyone at that time, there were too many girls at the school I went too and it seemed like everyone was cheating anyway so this just made more sense to me.