I can't even conceive of an endorsement for an engineer/producer, but I suppose like anything, they could exist. My doubt comes from the fact that endorsements exist to help a company sell products (especially to the public). Very rarely do you see a distressor
on stage, or "Big props to purple audio!" in liner notes.
Endorsements are tough. I know a number of drummers who are endorsed by Yamaha, Roland and Istanbul. They are very skilled, very hard working, touring musicians. With the exception of Istanbul, nobody gets free gear. Just better deals, and some special treatment (like resurrecting a discontinued finish etc.). They worked damn hard to get to a point where companies took them on, and they had
solicitation. They didn't e-mail out of the blue, nor where they approached out of the blue. They, through years of experience and networking, met the right people.
Endorsements are also not ENTIRELY a GOOD thing. There are numerous CONS to being endorsed!