ashtaron: That's exactly what I did with mine. You only need to cut a spare polycap in order for it to fit inside the square hole in the bare back of the MG Red Frame, so then you can attach the connector of the HG 1/100 backpack there.
In-depth explanation (image taken from yunamon's site)
Blue circle: you need a spare polycap and cut it or sand it in order for it to fit right in the square-shaped hole in the middle of the bare back of the MG Red Frame.
Red circle: you need some kind of bridge to connect both polycaps on the lower back of the MG Red Frame and the one of the 1/100 backpack connector. For that, you just need to cut from a runner a little extension. Given that it comes cylinder shaped it fits perfectly on both sides so you just slide it inside both polycaps. You may want to use a bit of white glue to reinforce it.
That way the beam saber backpack looks really natural on the MG, as the proportions don't change all that much and allow you to build the classic Red Frame instead of attaching to it the monstrous backpack the Tactical Arms is lol.
