It's a really interesting possibility, and perhaps Tolkien kept this figure in mind while creating the character. I checked the Encyclopedia of Arda, and it gave this description for the origins of his name:
Quote:
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In Unfinished Tales of Númenor and Middle-earth, Christopher Tolkien indicates that his father intended to change this derivation and bring Radagast in line with the other wizard-names Gandalf and Saruman, by associating it with the old language of the Men of the Vales of Anduin. No alternative meaning is provided (indeed, Tolkien stated that the name was 'not now clearly interpretable'). One possible source (a somewhat unfounded conjecture) would be Old English rudugást, 'brown spirit' (rudu strictly means 'red' or 'red-brown' - it is the source of the modern word 'ruddy').
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This etymology is not definite, but it seems to make sense. Maybe what happened was that Tolkien noticed the similarities between the two words
rudugást and
Radigost, and decided to combine their traits: the "brown" part from the Old English word (and brown implies a closeness with the earth like Radagast had) and the aspect of "tending" (although it's animals and birds Radagast looks after, not travelers and hosts).