Oliver,
I can only support you in your conclusions. When Willy Reschke published his JG 300/301 chronicle some years ago he included a foto of a Bf 109 G-10 from IV/JG 301 with a Rumpfband and a thin horizontal bar which did not fit to the known "regular" allocation of Gruppenbalken.
I had the pleasure to discuss this topic with Jerry Crandal, who had also noticed the strange useage of Gruppenbalken on Fw 190s and Ta 152s of JG 301. Based on that, we came to the same conclusions as you did. Due to the lack of published fotos we were not able to fix the exact time, when this strange group-markings were introduced, but we were quite sure, that it was common at the end of the war. Perhaps Jerry - has some new information which he will publish in his planed book on "Dora-Nine"
Greetings
Carl