Been going through the collection of kits.

[rickshaw]

The Germans also experimented with the R4M rocket equipped with a hollow-charge warhead.  Can't find a reference to it at the moment but it was used on the Eastern Front towards the end of the war.

[NARSES2]

According to various sources the He 177 was mooted to carry the Pak 40,

Beat me to it. I've also seen outlines for larger guns fitted to aircraft (how serious the plans were I don't know) that fired a counterweight to help deal with recoil forces. The Germans also looked at (planned ? tested ?) large calibre mortars that fired downwards whilst again using the counterweight method to counteract recoil.

As I said I've never been sure how serious the German's were with some of these ideas.