Last time tonight, after some digging realised my code could a little better for getting the path for the Cover Art into a useable format for Mastodon.py
For some reason, #VLC insists on passing a URI around, whereas #Clementine just passes a filesystem path.
Anyway, sorted out now, so that in the VLC version of nowplaying, I unquote a urlparse'd path and throw that at Mastodon.