Hello
Interesting question to which I was unable to find a written answer.
However, if you have a look at FCOM/PRO/SPO/25/30 ABN AND EMER PROCEDURES regarding dispatch with gear down, you will see that in case of a dual eng. failure you have to increase your speed up to 300 Kt to attempt a restart and it also says that you will have to disregard the VMO warning.
In normal conditions with the VMO/MMO switch set to "L/G DOWN", a VMO warning is only provided if 235 Kts are exceeded, I don't believe that it reduces the protection margins as well.
The bottom of the Barber Pole is computed by the FAC according to the airplane slat/flap configuration. The VMO/MMO speed computations done by the FAC do not take into account the landing gear position, however, an overspeed warning is probably available.
Standing to be corrected
Cheers