Back in 2011 there was discussion on the possibilities for one day. Plainly not possible.
But what about one year? Is it possible to travel in each direction (where appropriate) from end to end on each timetabled route/route number (including school diversions) available for normal public use during the course of a year? Excluded would be school only services and special event charters/services.
Sadly, even if the time and willpower was forthcoming, I think not.
The fly in the ointment is the unique Christmas Day services in the published Christmas Timetable. These can't be completed during Christmas Day, thus even a whole year is not enough.