Sure enough, there he was in the Key to Otherland opera. I would assume my lack of memory of Mr. Allmine is simply due to the fact that it has been so long since I've seen many of the classic Mister Rogers episodes. Either way, after watching episode 1425, my curiosity was piqued.
According to a comment left on a web site remembering Fred Rogers, "Mr. Allmine was the epitomy [sic] of selfishness and helped us understand those who would not share." TV.com places the appearances of Mr. Allmine in the years 1974-1975.
But that's it, friends. I could find nothing else about Bert Lloyd or his character Mr. Allmine. If anyone has memories, thoughts, information, etc. about Mr. Allmine, please feel free to leave a comment. I'd be curious to know more.