Not quite an alternate costume, and not for Xander, but:
When Willow looks at herself in Buffy's mirror, after putting on her ghost sheet, and before going downstairs, she idly thinks "I could be naked under this, and no-one would know."
She would later regret that thought.
This could go several ways. Perhaps she spends the whole time hiding in a bush, which itself would be pretty AU, and potentially disastrous. OTOH, perhaps she wants to hide, but her conscience won't let her ignore what's going on, so she tries to combine 'hiding' and helping, with much comedy over her failures, and the fact that clothes pass through her like everything else (because as TV Tropes puts it, <a href='http://tvtropes.org/pmwiki/pmwiki.php/Main/NakedPeopleAreFunny' target='_blank' rel='nofollow'>Naked People Are Funny</a>). I think the second one is more likely, but it could take a few screams before she starts to move.
Another version is that the spell affects her mind a bit, so either she doesn't know she's naked (and refuses to believe anyone who tells her; whether she realises she was naked after the spell ends is up to the author), or she knows, but doesn't care... until the spell is broken. :evil2:
The latter is used in another idea I might post later, but Willow being a naked ghost on Halloween is almost an afterthought, there.