Calling creatures with more than 18 HD

[Morph Bark]

With the greater planar ally/binding spells you can get a creature of up to 18 HD. A friend of mine recently told me there are ways to call a creature of up to 26 HD or even more, including unique outsiders such as Demogorgon. What ways are there to do this? How can you improve calling HD amounts? Is it possible to even call Asmodeus?

[Khatoblepas]

You can call Asmodeus, but it requires some serious magic. (http://www.d20srd.org/srd/spells/gate.htm)

Gate can call any creatures, including deities, but you can't control them unless they're non-deific, non-unique creatures.

Alternatively, Reserves of Strength, a feat from Dragonlance I think can uncap Greater Planar Ally.

[Aeryr]

Okay... let's break it.

At races of faerun, there is a feat, infernal bargainer, +2HD

Malconvoker has improved calling, +2HD

That is 22 HD with greater planar binding (original cap of 18HD).

Then there is Implore (9th level spell) in dragon magazine 336 that has a base cap of 24HD (28HD).

Of course there there is gate, but binding a duke of hell... is... awesome.