In general, we've avoided "extra action" stuff with Fate, even with things like the supernatural speed powers of Dresden, due to the rather warping effect extra actions have on a sense of folks getting fair "spotlight time". I would favor alternative approaches, though that's not to say someone can't come up with a clever extra-action implementation that works fine for a specific build of Fate.
Fred
http://games.groups.yahoo.com/group/FateRPG/message/20961 (http://games.groups.yahoo.com/group/FateRPG/message/20961)
I feel like this would take the sting out of having single bad guys just magically getting to go a bunch of times every round for no other reason than the GM wanted them to. Basing them on FP means the Big Bad has a tactical decision to make: does he act many times or save up FP for single, effective actions? Does he alpha strike with FP-boosted multiple actions or parcel them out, saving some in case he needs to make a retreat? Because he won't be able to keep up his cheatin' ways forever.
The best way to do this, done in other FATE games, is to give the boss multiple actions or make it a "modular" creature-- it's narratively a single creature, but is built like multiple characters and has multiple actions. A classic example is a Dragon. You'd build its' tail, it's maw, and each of its arms/claws as separate characters. They each get an action during each exchange (maintaining action economy) and they (probably) share stress boxes between them (at the cost of remaining in the same, or nearby zones as you'd expect). Stat the skills of each piece as makes sense.
Maybe each piece has different Aspects and players can declare which they're attacking for max benefit. "The tail has the Aspect, 'Not Being Watched,' right?"
Maybe the claws make grapples while the tail maneuvers or establishes blocks and the maw bites or uses a "breath weapon" of some kind.
Dragon (4-part)
Physical Track:
(Claws provide:) [][][][] (Tail provides:) [][] (Maw provides:) [][] (+ Extra for the Endurance skill rating each piece might posess)
etc.
Already tougher, and it hasn't even taken Toughness (on any of its pieces), yet.
Perhaps allow people to take out pieces of it as you go along-- after all, if they hack at the tail exclusively, it's gonna get chopped off / wounded beyond use, right?
Be creative -- Think outside the box.
Maybe the BBEG has a cloud of dark energy that surrounds him that lashes out at his enemies...
...maybe he has an enchanted sword or shield that protects him.