I just found this! Interesting!
Gruppen of JG 300 were probably operational for the first time in the Begleitjäger/Sturm role on or around the 29th May 1944 . The 8th and the 15th USAAF attacked the Politz refinery complex and Messerschmitt at Wiener Neustadt respectively but initial attempts to form a Gefechtsverband with IV./JG 3 for perhaps the first time under operational conditions were scuppered by a lack of radio communications. The Gruppen of JG 300 had no way of communicating with the Verbandsführer and their erratic manouevres contributed to the break up of the battle formation and probably the death of 100 plus victory ace, Kommodore of JG 3 and former Gruppenkommandeur IV./JG 3, Major Friedrich Karl Müller. Müller was forced to leave the formation, his machine damaged as a result of a collision and died in the subsequent crash landing at Salzwedel.
from:
http://ourworld.compuserve.com/homep...enmordens.html
That seems to suggest that JG 300 was indeed active against this B-24 raid - and of course its escort. However it doesn't make sense with regards to its supposed home base...