I can find three accounts of Luftwaffe intruder ops against the RAF and the USAAF, two regarded as successful:
http://www.baermann.biz/pauke/index.php?itemid=145
More Operation Gisella:
http://www.raf.mod.uk/bombercommand/mar45.html 3rd March 1945
Why not do this in 1943? Risk of German technology falling onto Allied hands perhaps?