Good article (as usual with your blog).
In answer to your question ...
Mortars were tried by III./JG 53 against Anzio in February 1944: 16 shells expended during the month (Ultra VL 8016).
I've seen Ultras — but don't ask me to find an example — from the tail end of the Normandy campaign and on in to the fighting in Alsace-Lorraine where orders for the coming day would include this or that
Gruppe being at readiness with mortars to engage ground targets.
In my Jagdgruppe 200 article you'll find four or five instances of them using mortars against Resistance targets in Southern France during July and August 1944.