Suikoden 5 had something like it as well. In fact, that'd be the preffered emulation method because SOME of those guests, though not playable, came with battlefeild or feild advantages(like being a mobile store/blacksmith, or regenerating everyone by 10 points per turn, etc.).
That could be complicated programming, though, so if what I just mentioned is a bit much the bare minimum in the first post would suffice.