Dentists are pretty much prohibited from recommending things like DMPS or DMSA because they are not doctors.

They can only recommend or give you nutritional supplements, anything else is beyond their scope and they could lose their license for acting as a physician when they are not.

Which means, in essence, don't even bother asking a dentist about how to chelate your mercury properly.

Most chelation doctors do recommend DMPS and DMSA even with metals still in the mouth, if the situation warrants it.