That said, you might argue that if you qualify some other way, say by being a Wizard 5 / Chameleon X / Incantatrix, you could advance the Chameleon with Incantatrix.
I don't allow that interpretation in my games, but I've seen it before.
Yes, that's the same logic used to justify dual 9s for the Chameleon (You get 9th (and 10th) level spells some other way, then add on extra spells to the Chameleon). The real question is, assuming it works, can you use a +1
arcane spellcasting class on the Chameleon, given that it's not always arcane.
To go off onto the tangent that everyone else is using though, you can definitely increase spellcasting using a Chameleon. From RoD, 112:
You gain the ability to prepare and cast arcane spells, which may be chosen from the spell list of any arcane spellcasting class. You prepare and cast these spells just as a wizard does, including the use of a spellbook.
Aptitude Focus's restriction states that:
It's only operation is on qualification for prestige classes. Once you qualify through other means, you can still advance Chameleon spellcasting, since they definitely do get spells. The trick is whether you can advance specific Arcane/Divine spellcasting progressions and, if so, how that works.
Like, let's say you can advance spellcasting, and use a +1 Arcane spellcasting class. Does that class advance the Chameleon's spellcasting generally? Or only the spellcasting for the purpose of the Arcane aptitude focus? I'm looking for rules based arguments, rather than common sense ones (Think of it as a black letter law kind of thing).