never tried it, but it should be quite as easy as you think of those FFs where the Summon can't be attacked.
Just reflavour for example fireball as a full round action after which "ifrit appears and throws a ball of fire at your ennemies", it should do a little bit more damage or ignore 5 points of fire resistance as it's longer than the usual standard action.
So the kind of "FF summons" you have in mind actually needs nothing from summon monster. It's quite impossible however to create those FF summons that replace the group, like FFX for example as your group won't disappear for a few rounds, well you could do it with gate that gated you to the plane of the summoned monster at the same time it comes to you, but that will be very weird.
I honestly thought this was how the Summoner class was going to work before I saw the beta.
I could see it being an archetype variant that gave up the long duration summons for this in it's place.
The Eidolon would do the disappearing on the round such a summons comes into effect, coming back immediately the next round (unless you did another summons).
I'd probably introduce this kind of magic this way, less impacting to the game as a single character's ability instead of magic anyone can learn and use.