Is Chameleon's spellcasting compatible with Sublime Cord? That's the only way I can think of expanding the spellcasting.
I don't think so. The Chameleon's casting cannot be used to qualify for prc reqs, so you would need to have 4th level casting in something else and then apply it to the Chameleon instead. Which is possible, but probably not feasible in a pre-epic campaign
I speak AD&D to my fluff, 3.5 to my crunch and 5E to my players.