For your paticular situation, the closest examples I can think of are,
A) When a maximized spell is also empowered, the damage is max + 1/2 normal rolled damage.
B) With the weapon master PrC, they can max damage any normal hit, but roll crits just like anyone else.
Not much for presedence, but I think 24 + 4D6 is the way to go. Nothing in 3.x lets you multiply a multiplier, but the maximize should affect the base spell.
"Now, personaly, I think either 1 or 2 are valid options (I've played with both ways of dealing with crits), and I think 3 does not work, because you're saying that maximize isn't effecting all the dice. The crit chance is part of the benifit of a spell that requires a to hit, so any metamagic that effects damage should effect the crit as well"
The only problem is is that options 1 & 2 are not how crits work in ANY situation, and 3 is closest to the way they are handled RAW. If they worked the above way, a maxed empowered fireball would do 90 points, not 60 + 10D6/2. Maximize does not effect all the dice all the time.
It is something that should have been adressed after five years of this possability floating around, but there is always a hole to be found.