Finally something to put up to test.
The attached file will add a source
Epic Level Handbook - Leveling
A condition "Epic Leveling" will be made available on the Adjust tab for the character. There will be no obvious changes until the hero has 21 or more Hit Dice from race and classes. The Hero.Epic tag should automatically be applied to the hero at that point (no need to check the box).
If you check the condition though, a series of eval scripts will run to determine the hero's BAB and Saves at the 20th HD and apply the epic progression for attack and Saves for all levels after that.
Known Limitations:
This setup assumes the hero will achieve 21 HD within 30 class picks. I know it seems obvious, but one of my current characters did not get to 21 until the 27th class pick. Templates can be a convenient way to add home brew campaign bonuses to your hero, but they count as class picks as well. If you find that your hero may not reach epic until after 30 picks, let me know and I'll expand it out further.
In addition, I am unsure how well this will handle HD added from sources other then the base race and class leveling. Again, if you notice problems let us know. I cannot guarantee to be able to solve it, but it is likely someone on the forum will be able to find a solution or workaround.
One last point, this uses a bunch of "findchild" statements. I did only light testing so far and my home computer is decently powerful so I did not notice lag. Your mileage may vary.