There was IIRC an upwards firing mortar type mount in a fuselage installation tested on the FW 190. This was for day use against bombers however, and was triggered by a photosensor as the fighter flew through the bombers optical shadow......
In addition I recall that there was a wing mounted vertical rocket system also tried . This however fired downwards as an anti-tank weapon. Work was certainly done on triggering similar systems by sensing the tank's magnetic field. It all never sounded exactly habit forming to me!