Dragon Magazine is, unfortunately, not allowed. Otherwise the idea would've been excellent. And the very point of Cosmic Descryer is to get Cosmic Connection and that requires seven levels and I am not exactly certain about us ever leveling six times during the life of that PbP.
The +4HD cap is actually alot more useful unless you are not really all that interested in summoning, especially when paired with the Malenvocer. The huge caster level increase is not all that impressive with only one spell a day, especially when the PrC puts you 4 caster levels behind to begin with (unless you are not taking any other dead levels and picking up practiced spell caster). If you really have access to all the book sources, things like Suffer the Flesh persisent (Magic of Eberron +5 caster level), orange ioun stone (+1), ring of arcane might (+1), and Create Magic tatoo (Spell compendium +1) can really boost you up. Not as much as the Cosmic Descyrer, but +8 on every spell all day is not bad.
You could be right on both accounts. I am rather set on War Weaver though, I am really interested to try that. However, is it worth it assuming Uncanny Trickster/Legacy Champion/bloodline trickery doesn't work? At level 30 5th level buffs are not quite as impressive as they are at level 15. Makes me wonder.
Bloodlines works as Omen of Peace noted, the other PrCs are up to your DM and are cheesy. But even with a major bloodline, that gets you to 8th level spells (assuming war weaver 5). Sanctum spell metamagic lowers a spells level by 1 when cast outside your sanctum (not slot, but level). If you use this feat in conjunction with War weaver you could get level 9 spells. The key to your build then is to just find other classes that can benefit from the level boost. Abjuration champion might work for example, but the AC boost is not really worth it at those levels. I am sure there must be something that could benefit from the boost.
I'd like to add support to those suggesting some Dweomerkeeper levels. The ability to cast a spell as a supernatural ability means not only free wishes, but avoiding the nasty, nasty drawbacks of epic spells that you should create. No 50 k xp payment for me!
I did not realize you could use that with epic spells. I went back and looked, only metamagic and feats that modify spells do not work with epic spell casting. I wonder what other fun class abilities can modify epic spells.
It is still too bad though that Eldrich Master is out. I always thought Blackguard 22/Eldrich Master 8 would be fun. Epic Blackguards can convert all the levels they want (assuming you fall once epic). Eldrich Master would grant 5th and 6th level spells. A few epic feats could grant 7th through 9th. Plus you would end up with the cleric and wizard spells lists.