What are the chances of me taking Tom Daley (diver) on a history tour around Sheffield?

Sorry but I think your initial assessment of it being a pretty tall order is the most likely outcome. You'd be better off doing up some leaflets and handing out a set number of flyers to see if anyone turns up for one of your talks. You'd also be advised to get the tourism guys on board too so someone in authority knows what you're doing. Otherwise, I can see someone complaining and you ending up getting arrested, despite your good intentions.