He (Mallory Earl) co-produced an album by Hot Tuna in 1975 - that was a band featuring Jorma Kaukonen and Jack Casady from the Jefferson Airplane, but that's hardly a fantastic thing to have in your bio.
Googling creatively I managed to come up with a few albums that he worked on as an engineer or producer, some before Frenzy some after but none of them very noteworthy except maybe The Grateful Dead. I got the most hits googling: "Mallory Earl" engineer He only worked with Steely Dan in his dreams, though! I think his widow is referring to "Jay and The Americans" who Becker and Fagan worked with in their pre-Steely Dan days.
Nothing about him working with Hendrix (other than the odd Enz related hits describing him as "Hendrix's tea boy". "Tea Boy" was a term used to describe the assistants in recording studios in England back in those days. Sounds like Earl took it a bit literally in relation to Hugh Padgham's role on Frenzy.
History has had the last laugh. Hugh Padgham went to become a very successful producer with loads of awards and Mallory Earl languished in obscurity...