My friend recently lent one of her old homecoming dresses to me for this years homecoming. While my family couldnt of spent a lot of money on one, I probably could of gotten one if I really wanted it. I decided at the last min. to go, and didnt have a date, so it wasnt really a big deal. Another friend and I went over to her house and we all tried on all of her old dresses, we found the one we liked the best and she let me borrow it. I was very grateful! The dress is at the dry cleaners now, I'm going to pick it up and return it tomorrow! I have let her borrow a couple of shirts and stuff if she needed something to match a certian skirt. I'll remember she let me borrow the dress, and help her out if she ever needed to borrow one from me.
Anyway- to answer your question, I dont think you are at all being rude, unreasonable, or any of those things. I would be very apprahensive (spelling?!) to allow my DD to loan such an expensive and special dress out to a girl I didnt know. (no, I dont have a DD, but if I did, thats how I would feel

) I agree with the person that suggested you call the parents and just say that you dont mind thier DD wearing the dress, but that your younger DD's would like to wear the dress one day, so you wanted to make sure that you got it back, or something to that affect!
That way your DD wouldnt feel bad about having to tell her friend no. Good Luck!