Tried to search for this and talk with industry experts, but no conclusive answer so far....

A General Twin Seal valve, or a similar single body double block and bleed valve, often uses a device to relieve the pressure in the cavity between the 2 seals to the upstream (or downstream) side of the valve.

Is this device a "pressure relief device"? Does this device need to be inspected or replaced on an interval? Does anyone know of examples where this mechanism failed and caused overpressure of the valve? How would this overpressure most likely manifest?

I have several locations where I would like to install these valves to solve leak through problems in isolation, but I am wary that I will be solving one problem and creating another one that is not so obvious.

As operator, its your choice of what to call it.  In 1993 we replaced actual springload 3/4" PSV's with a check valve that had a 25 psi differential spring in it.  We tied the outlet to one side of the valve that had a thermal protection system on it or was a buried line not subject to thermal expansion. We worked with Foster valve to tap the flange of thier DBB (SAF T Seal knock off) valve and adopted that valve with the check valve attached as our standard. Our maintenance proceedures called for a an annual or semi annual inspection of the valves (grease, verify movement, corrosion inspection, and we pulled the check valve to verify it was not "stuck and still working".  We did not cailbrate the spring pressure mechanically, but our inspectors knew about how much resistance the 25 psig spring had in it.  The valve bodies were either ANSI 600, 900, or 1500.  I think we may have set the 1500 to 100 psig differential.

Bottom line, we got out of the annual or semi annaul relief valve testing per DOT 195, 192.  We also have no move venting when and if there was a thermal relief or a valve failure.

There rare cases of thermal overpressure on a DBB are very rare.  I would even guess it would be nearly impossible, but you still need to have the system.  The rests could be as catastophic as a valve body failure or as minor as loosing one of the seats. 

Last year I went though an exercise to review the installation of PSV