As said, I would not allow the Incantatrix ability to work on epic spells. The class was designed for level 20 and under. Much like I would not allow, say, a Loremaster to grab epic feats with their bonus feats. Not up to my ruling through, so I wish you the best of luck. Someone has to win this thing eventually!
I wasn't talking about using the incantatrix ability to modify the epic spells.
I was talking about the fact that to mimic any metamagic feat the incantatrix could add to this epic spell would be at most +150 to the spellcraft DC, meaning it would be trivial for any 21st level mage to get around it.
---
The problem I see with giving people 400HD and 18 Divine Ranks and then banning things on a case for case basis is pretty weird. With this amount of power, any given problem can be solved in a myriad of ways. Banning the simpler ones (Incantatrix doing the Metamagic for you, Transport + Foresee working), is just speedblocks.
Here's a bunch of the objectives and the easiest solution, as I see them. Every time you ban the easiest solution, it doesn't mean the problem doesn't have to be solved. So an optimizer needs to use his HD+Divine Rank + Epic Spells (+ Skills + Feats) to find a work-around, if you keep banning work-arounds the task becomes meaningless, because all you do is setting the bar lower and lower for what can be used.
#1 Teleport to Dispater and get around his defenses;
Easiest Solution: Transport + Foresee + Some Ignore Magical protections Epic Seeds
#2 Be immune to any preliminary defense Dispater has set up;
Easiest Solution: Demilich + Planar Bubble + Epic Spell to make you immune to being timestopped and Disjuncted. Another Epic spell to make you immune to epic spells (I'm guessing you need one per epic spell, so this takes some time).
#3 Make sure Dispater doesn't escape;
Easiest Solution: Contingent Transport + Foresee + some ignore magical protections epic seeds to follow him.
#4 Kill Dispater;
Easiest solution: Trap The Soul from Demilich, and a few retries.
All of these can be bypassed by the SDA Overlord: Abominations. But reading the "how to create your own deity" chapter hardly qualifies as optimization.
All in all, a DM could keep banning these things, and an optimizer would keep needing to get a new work-around. This then develops into a what will the DM allow, until one of the points become impossible, and the task thus becomes impossible, or a work-around is allowed and the task is completed.