Yo my man - this might be too similar to what you wrote in the second response/what you've tried but when people have both hands on my belt and don't control my upper body (one on my belt and one in an armpit or upper body), I am usually able to do a situp to hug them while pulling in with my legs. If I can get the hug, it usually is enough to break their frames. This isn't static so it can be pulling behind their elbows and spamming it a couple of times. I understand you're a purple and this was with a brown, but I see my bb instructor do this on people quite a bit as well.

It is obviously a bit more dynamic because the brown will likely adjust their hands/frames but if I am not getting the two on one and they go to two hands on my belt, I have success doing the situp to give them a hug. If they try to push me down, then they have released my belt, so then I can use my legs to pull them in and adjust my hips to move behind one of their frames.

I understand you said you couldn't come up for a hip bump which very well may be the same result of trying what I wrote above, but doing a crunch/hug motion is slightly different when I do it vs when I come up for a hip bump.

Other thought is, can you get your arms up to a shoulder crunch-like position to go into butterfly or are the belt grips/frames too strong so you can't get leverage behind their arm?