IMHO passengers have a weird expectation of what lav facilities are on board an aircraft. I have had passengers get on the aircraft and use the loo before take off! and that is after 10 minutes in the FBO waiting room. In the glory days of 9 passengers in a Chieftain for 3 hours the passengers did not expect a loo so they acted accordingly. Why do they expect a loo in a Mustang that has a smaller cabin and is able to stay aloft shorter? Perhaps we should do a MOL and charge the customers for the lav service. When they saw the bill from the FBO I am sure that next time they would take an extra 90 seconds before take off to make themselves comfortable. Bills for Lav service run £50-100 which may not be a significant sum on a GV but is very significant on the DOC of a smaller aircraft. I guess the Mustang DOC is about £500/hour so if the loo had to be serviced after every sector their DOC could go up by as much as 25% (assuming 1 sector = 1 hour).
I think it is a case of passengers expecting a loo in a jet regardless of size. Loos in small aircraft should have a large sign. "Emergency use only, it will be the most expensive p you have ever had"