I've written implementations of Salsa20 and ChaCha that accept 384 and 448 bit keys. It was fairly trivial to implement, the 'sigma' constant is replaced with key material, and the counter, (which was initialized at 0), rotates and can be initialized at any value. So along with the 64 bit vector, the entire 512 bit state can now be initialized via a key/vector.
This does not affect the performance in any way, and if a key is constructed using key + sigma + 0, the output aligns with an implementation that uses a 256 bit key. Provided that the key used is created using a strong PRNG, and the values that replace the constant are sufficiently asymmetric, this should not present a problem, as the constants function seems to be purely to reduce symmetry.
Is there anything incorrect in the description given above?
One other question, I am considering extending max rounds to 24, I have looked at the mathematical model, and it seems that this should not do anything other than further diffuse the input, is that correct?