Yeah, Tactics Ogre does just that on PSP. You press a button and suddenly the field becomes top down, making unit selection easier (even with physical controls it can be annoying sometimes). Mercenaries Saga 2 does have a zoom function though. It doesn't help a whole lot, but it helps a bit. One tip I can give is to tap right below the tiles instead of tapping in the middle of it. If you want to select units, tap right above their heads.