Girls go to school to get their RMT so they can earn a living without being MPAs.
If they get caught providing 'extras' they will lose their designation and, from my experience, they tend to take it pretty seriously.
It is not hard to find MPAs that provide RMT quality massage, especially the Chinese traditional massage/tui na places that have been well documented here.
No doubt, you could smooth talk an RMT into extras, but what's the point when there are 'professionals' out there who are willing and able to provide both??