The quick answer is there are no near-term plans to have a native WACUP client for linux (or other platforms for that matter).
The reasons for it are that we first need to get to having an all WACUP client that doesn't rely on any (or at least most of the remaining core) of the Winamp 5.666 install that is made use of currently. Even once that has been done, there's then the time needed to port to an OS that I don't actively use & as I've not really been looking at going to other platforms, that's more code work than just getting to an all native Windows version.
So I fully appreciate that there is a demand for linux support but WINE 4.0 with some configuration does seem to be a reasonable solution for a native Winamp/WACUP experience but with the compromises that comes from not using Windows natively.
-dro