Here are two more ideas I tried to develop. I'm not fully happy with them, but just though of sharing them anyway. Whatever I come up with I don't seem to fit unicycle at all. I came up with the idea of it being an ancient item (yet again, one of the many versions) or maybe the historian is just cycling around with it or he just has a wheel, unicycle robot. This way the unicycle will the a part of the character. Before I decided that the historian is a historian he was just an adventurous robot and everything that happens afterwards is just sequence of random events. It would make sense that a traveller moves around with a wheel... I'll try to develop further my other ideas.
P.S. - Please excuse the grammar!