Well, I couldn't find it, But have got so damn close to it, You can summon in battle, Dismiss in battle, and when the battle ends, you get your normal party back, But only Arshes/Aluxes battle sprite shows, Also, I can't figure out how to make THEM get the exp, Here's a demo, See if you like it so far:
http://www.box.net/shared/2fnv6e6c55
I think I'm on the right track to what you need though, right ?
P.S - The Summon starts at lvl.20 (It needs to be strong, Lol), and it can Lvl up, Although you don't see this on the Menu or anything, Pretty cool I guess, if you don't want it to Lvl up, You could always just change the Max Lvl of it to 20, But that'd be no fun on the powerfuller Summons, Lol.
Another thing - Hilda is the one that can Summon on the Demo, and don't laugh at the Summon, it's just the first thing I clicked on, Lol.