The access panel could be made of 1/2" drywall with the opening surrounded with casing to match your doors and windows. A creative panel could be covered with padding and upholstery or fabric, but you may not want it to be 'out of place'.
If you could use a fibered grinder blade in the saw it may cut the plaster a little easier. Either way, there will be plenty of dust. The dust can be minimized somewhat by holding the hose of your shop-vac in front of the saw. Also, a reciprocating saw with a Milwakee Ax blade is quick on plaster.
Be sure to wear eye protection and/or full face protection along with a dust mask.