Asha'man said:
This level of shameless abuse would, however, be curtailed in any actual game (as opposed to theoretical discussion) of D&D by the simple fact that Solars (and to a lesser degree, Couatls) exist in finite numbers with positions and duties in celestial hierarchies. Under no circumstances would the Heavenly Hosts allow you to co-opt its foremost champions for your own benefit.
Actually, Solars show up on the Celestial Encounter Table in the Manual of the Planes (3.0). The Celestial Encounter Table is used for the Twin Planes of Bytopia, which is doubly infinite; with a nonzero Solar density on an infinite plane, there actually ARE an infinite number of Solars.
But yes, the cheese runs freely when you do that type of thing, and the DM should lay the smack down on a player who tries this (or similar - Planetars are actually the most efficient, at CR 16 casting as a Cleric-17; Permanent Summon DC 210; you can grab them at day 10, warming up with Couatl's; and they get two 9th level spell slots for you to abuse; Leadership is the more commonly cited version of this - low-level NPC classes? Adepts get 1st level spell slots at level 1, and are an NPC class; with a good Leadership score, and the feat, you can quite easily be Permanently Summoning outsiders on day 1).
There's a couple of basic house-rules that prevent this type of abuse (any one does the job):
1) You cannot mitigate an Epic spell down below some fraction of the spellcraft DC before mitigation (e.g., 1/4th; that Permanently Summoned Planetar can't be done without a spellcraft DC of at least 52... with the attending 468,000 gp, 18,720 xp, and 10 days to research).
2) Some part of research costs is calculated before mitigation factors (XP is my favorite here; sure, you're "donated" spell slots brought the spellcraft check from 210 to 0... but you still need to spend 75,600 xp to research it. Have fun doing so before level 40).
3) Summoning variant: It's the same one each time. When you Permanently Summon one Planetar, and try to summon another the next day, the spell returns "not available". If you research another spell to get two, but still have the first one out, you only get one (as the other is currently in use).
4) No. Just.... no. (technically not a house-rule; per RAW, all Epic spells MUST be approved by the DM).
5) Stuff I didn't list (e.g., Divine intervention).