The answer is yes they could be jettisoned, but with extreme caution because doing so with anything but empty tanks could jeopardise the airframe. There is an apocryphal story that attempting to jettison full tanks would result in them staying where they were and it was the wings that were jettisoned.
Each tank could carry 260 gallons (approx. 2000lbs) of fuel each.
You'll notice the triangular warnings on both the support and the tank, indicating to ground crew that caution was needed when adding and removing the "overburgers".
According to the Lightning Aircrew Manual the procedure for fuel dumping was:
A guarded, two-position OFF/FUEL DUMP switch is on the left console of
the F Mk 6. Fuel is dumped through a pipe running from the bottom of
the centre compartment to the aft end of each overwing tank. The pipes
are sealed by a blanking cap held in position by a hinged lever
connected by two explosive bolts. When FUEL DUMP is selected, the
explosive bolts of both tanks fire to free the lever which hinges open
under the influence of a spring. Air pressure then dumps the fuel
overboard, the transfer in the tank to its centre compartment
following the normal sequence. From full, the tanks take approximately
two minutes to empty, and a further two minutes for residual fuel and
vapour to clear.
It later notes
Overwing tank fuel may be dumped at speeds between 220 knots and the
maximum carriage speed at heights up to 36,000 feet. Dumping is
normally to be performed in straight and level flight but turns and
not more than +2g may be made if necessary.
Dumping was a necessary pre-condition of jettisoning the fuel tanks. The manual doesn't provide any emergency scenarios in which such an action might be necessary, but does say:
Empty overwing tanks may be jettisoned between 200 and 250 knots and
up to 0.75M at heights up to 40,000 feet in straight and level flight.
Overwing tanks containing fuel are not to be jettisoned.
Here is XN725 during testing of the tanks, with large fins to ensure clean separation under various loads.