That's how ferry pilots and freelance instructors work. They don't have type ratings for every single airplane ever made, or else they would be out of a job.
I dont know what ferry work you have done but the big problem is insurance requirements.
On the Jet ferries I have carried out they want to know everything about me to act as a Captain! Copies of licences, medicals, Type ratings, recurrents hours on type ,total hours, crossings etc etc etc.
Dont think its a matter of jump in anything and off you go? I think that used to be the case years back but certainly not nowadays,
Pace