If I understand correctly, the heavy lifting for your soundproofing is handled by the DD+GG (the damped mass). I've seen several variations of the window plug, but I get the impression that the soundproofed plugs are intended for people that need access to the window. In your case I'd be temped to just fill the cavity to prevent any resonant issues. If that is correct, then you can fill that window cavity will f.g, or foam boards, etc, etc. 3/4" is not much space to get any significant absorption.
If it was my window, I'd probably fill it will 3/4 foam board since you are planning foam board over ithe cavity anyway. Just make sure it's attached well so that it doesn't vibrate back there. Then put your f.g. batts in the wall cavity as you mentioned for absorption. If you need to be able to get to that window, then just ignore this