Hello @rboss,
Thank you very much for your detailed contribution. I'm sorry, that I didn't tell the whole story in the first place.
The above problem description relates to the [WS] Deezer, that I had reworked and completed with configuration settings and other extensions two month ago.
Meanwhile I detected, that the cover preview feature in this last release of the Deezer WSS only works with (real) searches for artist + album and artist + track. The other scripts, that use an album/artist/track-ID or an URL, never show the cover when clicking on the preview URL. This is because these scripts don't need to search for something and consequently don't have a ParserScriptIndex block.
My suggestion to allow writing acess to the output buffer "CurrentPreview" in ParserScriptAlbum would be the easiest way to clear the issue, that the preview URL can't be changed or set. The workaround you suggest via using a "fake" search for an album with a one hit result surely would work, but this involves much more programming work and computer resources. Also the user surely would notice this two step process, because the latest Mp3tag now displays progress bars for both phases, the retrieving search results and the retrieving metadata.
I mentioned the Bandcamp WSS, because the bandcamp server requires javascript for performing searches and Mp3tag doesn't support javascript. Consequently for Bandcamp the one hit result search workaround is not possible.