Changelings have the Racial Emulation feat to let them count as other races for qualifying for things. Goliaths and Half-Giants have Powerful Build to let them count as Large in several circumstances. Many classes and races let you ignore some requirements for certain feats/classes. And lots of characters can use UMD to count as something they're not for using, well, a magic device.
Is there a way to count as Evil in alignment for the purposes of qualifying for feats?
Anything with the evil subtype will count as evil for “effects” regardless of their actual alignment. Unfortunately, the word “effects” is not defined for D&D 3.5e—so we don’t know exactly what it means. Does it include requirements? Unknown, but it might. I have certainly never had a problem allowing it to do so. Anyway, assuming that this works, polymorph has “The subject’s creature type and subtype (if any) change to match the new form.”
Likewise for the other alignments, and there are options aside from polymorph for getting subtypes. Savage Species even has a ritual for gaining a subtype permanently.
I am not aware of any other option for qualifying as evil. Fiendish Codex II: Tyrants of the Nine Hells has the hellbred race, which can use evil spells and magic items despite not being evil themselves, and Complete Scoundrel has the malconvoker, which can cast evil conjurations even if they are a good cleric, and without it counting as an evil act, but that’s it.
