Audacious supports classic Winamp skins so if you're only after the aesthetics just use that. Alternatively you may be able to run the original Winamp through Wine.

I wonder, would publishing a diff be against the license?

I don't believe so, they can't control what you create or what others may do with it for personal use. They may only control winamp/their interests

The assumptions being, users of the patch get their own source and this isn't for commercial purposes. This is The Old Way

I think this is how we got MP3 support at large before the patents expired or something. Memory is failing me.

XMMS2!

I used to do plug-ins because it scratched an itch to code & provide things that'd be of use to others which when you've got a job to pay for that free time then it's less of an issue. Also the time of winamp & the plug-in / skin community of the time were somewhat reflective of how interested people were in tech vs how commoditised it's now become & generically boring imho. And I'm probably guilty of the change in behaviours with how I'm trying to fund wacup but 2 decades on is a long time & most of those early creatives have bills to pay.