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? It might, and I have certainly never had a problem allowing it to do so.
The basis for the claim that “effects” might include prerequisites, that alignment subtypes might be good enough for requirements, is that there’s one case of “effects that are keyed to alignment affect you as if you were” explicitly applying to alignment requirements:
Ordered Chaos
You are an unusually lawful Abyssal heritor
Prerequisite: Non-chaotic alignment, base Will save +4.
Benefit: Your alignment is not affected by the Abyssal heritor feats you possess. Spells and effects that are keyed to alignment affect you as if you were chaotic, as well as your actual alignment. For example, you become immune to spells such as chaos hammer and word of chaos, you could wield an anarchic weapon without fear of gaining a negative level, and you could take the Primordial Scion feat despite its chaotic alignment prerequisite.
(Fiendish Codex I: Hordes of the Abyss, pg. 86, emphasis mine)
The only part of the actual rule text that might make the Primordial Scion example accurate is “effects,” since it’s certainly not a spell and definitely not your own alignment, which are the other two things that the feat touches on. Rules-as-written, there are some problems here, since it’s only in an example—secondary source—but since we don’t actually know what “effects” means, we can’t be sure there’s any contradiction. If “effects” can include requirements, then there is no contradiction and the example is valid. That, to me, puts the burden of proof on those claiming “effects” doesn’t include requirements to demonstrate that, since I come from a base assumption that the rules are generally written correctly until proven otherwise. (Of course, they are demonstrably incorrect in numerous places, so this is far from the only reasonable baseline assumption.)
Anyway, assuming that this works, it works for all alignments. As for getting the subtype, polymorph has “The subject’s creature type and subtype (if any) change to match the new form,” so that’s an easy, core option. There are others, though, that might be better. For example, 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 for evil. For non-evil alignments, there’s also the aforementioned Ordered Chaos for the Chaotic alignment, which should work even if you don’t buy the argument about inferring things about “effects” more broadly from it. As far as I’m aware, Ordered Chaos is unique, however.
