Well, you can summon a Celestial Unicorn Cleric 7 with a Summon Nature's Ally spell...
Yeah, but that's still (summon) and not (calling)... Summon Nature's Ally doesn't leave behind a usable corpse.
Even with BoED allowing for calling advanced HD or leveled outsiders, I think as a DM I'd require that you know of the target you want to conjure up... perhaps some research, gather information, or calling a bunch of average outsider HD targets and interrogating them?
I also disagree with K's take on rebuking, but the rest is mostly golden IMHO.
I like spellstitched Revive Undead, but I prefer it on a *very* loyal minion (one that could use it on me in an emergency).
Spellstitching is very much like magic item creation IMO (note that it requires Craft Wondous Item)... as such, I allow the character doing the stitching to benefit from others helping out; much like with normal item creation. (YMMV)
Also, you don't have to be the one crafting the spellstitching on your own body... Hire an NPC to do it for you. This way you get access to spells you don't normally know. Also, if you are rolling in the gold, pay the NPC to use his own XP (at 5gp per XP).
Finally, spellstitching *must* be done by a wiz/sor. This may imply that the spells should be Arcane... but the actual restriction is that they must be from the conj, evo, or necro schools. If your DM buys into treating it like magic item creation, feel free to invite a cleric along for the crafting and check out the Divine spells from those schools.
Minor Note: The HoH version of Plague of Undead requires Black Sapphires
About recruiting leveled minions... I'd think it would be next to impossible to find the perfectly optimized 20th level NPC that has the exact mix of a few base classes and a half a dozen prestige classes that you are looking for. However, the opposite end of the spectrum should be much, much easier. Finding folks with one level in a base class that you want might not be that hard depending upon your DM. Retraining Feats or Psychic Reformation could tweak them a little.
If that is the case, then you might benefit from the Compilation of Low Level Builds thread:
http://brilliantgameologists.com/boards/index.php?topic=1444.0I had a funny write-up somewhere using a skeleton dire elephant with a stronghold space built into the ribcage. Stuff it chock-full of necropolitans. It would contain mostly level 1 Dread Necromancers to constantly heal it every turn. A few arrow-slits would allow for ranged attacks from a mix of level 1 Incarnum weilders using Dissolving Spittle or level 1 wiz/sorc with Precocious Apprentice and Fiery Burst feats.
I'd be willing to share more if anyone wanted to help me refine the idea...