You would want to make sure that you have the last filling removed before beginning any chelation.
Having it done by a mercury free biological dentist can minimize the exposure if following protocol written by Dr Hal Huggins.

Mercury metal does not have magnetic properties (diamagnetic) so it may not be attracted to a magnet (however mercury may be in many forms in the body from its metal state to organic forms).

Supplementing with activated charcoal or even chlorella before and after the appointment can trap some of the mercury that may have been swallowed during the procedure, although much of it may be inhaled during the drilling (that why an external source of oxygen would be ideal).

From a personal experience, you want to make sure you chelate slow enough not to overwhelm your body, because it may go on a rollercoaster during the process after decades of mercury poisoning.

Mike