Would it be possible to add an extra field to the ID tag for 'Remix by' and 'Featuring'. Thus when searching for a particular artist It would return their Remixes or Co-productions with other artists.

I can almost guarantee some mechanism for getting this done appears in a future 2.0x release. Peter and Roger are big fans of genres which involve remixes, co-productions, and the like. And now that it's EOL, features that the programmers themselves want are much more likely to be on the front burner. There have been several methods suggested, including using ID3v2's support for multiple artists and/or the other ID3v2 fields, but no specifics have been mentioned yet.

There are older threads somewhere around here discussing this possibility, including some with piqued interest from the Empeg guys. So don't hold your breath, but I would think this one has a good chance of happening soon.

My work around has been to, for example, place any remixes done by Portishead into my Portishead playlist (seperate from any album playlists). Doesn't work for searches though... and takes time to keep up, but it works.