Will be very difficult to manage the rule.
Pax far away from lav could make several attempted journeys only to have to return to their seats when people closer get there faster. Could increase air rage in some cases but would be good for preventing DVT with all the added exercise. F/As will spend a lot of time marshalling folks around and dealing with the irate ones.
Perhaps they could introduce a lav reservation system?
"23A - you're next - you've got 4 minutes - 43G get ready"