Well, according to Peter Straker personally:
Freddie did lots of bits and pieces on a all of the album This One's On Me. He layed down an amount of basic backingtracks. Some off them where later on 'properly' recorded by other musicians, some bits and pieces where left in. Basically Freddie was around all the time, doing things spontaniously -little bit of piano here, little bit of a handclap there-.
Again: a lot of these things did not end up on the endproduct. Some of them did.
Freddie is mentioned as producer (which is basically indeed what he did - this sort of activity is pretty common for any producer, though on most products -especially by bands- all the producer-played things are re-recorded. With solo artists lots of times bits of producer-music will remain). Not as a specific musician because in the end there where small bits and pieces everywhere and no-body could remember what and where was all done by Fred. (But, for instance, if I use my ears: Listen to the piano on "I've Been To Hell And Back"...!! Or the choir on "Heart Be Still"... Surely he is in there!)
Nor did they bother to make a list, they just stuck to the 'prouced by'. Which was also good enough for Freddie. He did this as a service to a friend and wanted to grant Peter a career of his own and not one that was pulled by the famous Freddie Mercury.
(And I'm sure, money was an issue as well... that's just how it works in the musicindustry)
This is what Peter Straker remembers.
(And that's about it - the guy is certainly not living in the past and does not want to think about this closed book too much anymore.
Which is, I think, a possible reason for this info not to be in the Freddie box for instance. Peter Straker is no longer in contact with anyone Queen-related)and besides: sessionmusicians are seldom granted credits. When you do session work for a band, the booklets even want to make the masses believe, it was ALL the band themsleves... Sad but true - the industry...)
And there you have it!