A great idea. I remember there was a free to enter friendly vs West Ham back in April 2002 around Easter time, there must have been a crowd of a few thousand there for it, had a great time. (We lost 3-0 from what I remember, but nontheless it was a good way to spend the evening. Bairdy played.)
Even if it's only a few hundred paying 3 pounds or so to get in it would make for great experience for the lads, and if it's against someone actually rather good (Barca or AC Milan Reserves) then we could no doubt get a decent crowd for it.