But if the mortise and tennon were well tapered, they 'should' part easily.
Agree, they should be tapered side to side and front to back or conical, but the mortises need to be perpendicular to the parting plane or the foam will be pulled out when you remove the barrel part of the pattern.
Also it will take some care to ram the mould so that the sand is tight enough to hold but you don't damage the foam parts.