One of my favorite 'Doctor Who' bloggers, Combom, has posted a link to an audio-play about the Fifth Doctor. The story's title is "The Haunting Of Thomas Brewster" .
I haven't heard it, but I'm fairly certain it has nothing to do with Tom Brewster of 'Sugarfoot'! But what a crossover that could have been, right? And Sugarfoot had enough "book-larnin'" and an open mind willing to accept the many wonders he might have seen of the Universe, had he tagged along as a Companion.
I'd definitely put 'Sugarfoot' in my list of "Other TV Characters as Companions".......
To have done such a crossover "realistically" (as if such a thing could be found at Toobworld Central!), I suppose we'd have to have Tom Brewster flitting about Time and Space with the First Doctor. But I think he'd get along much better with the Seventh, for some reason...... BCnU!