Have you ever tried binding?
I do it to myself sometimes, but because my breasts are quite small I don't get as much sensation out of it as maybe a sub with larger breasts.
I tore an old bedsheet into long strips, and wind them around my ribcage, from armpits to floating ribs (the end of the ribcage). Inside the wrapping, between the skin and the first layer of wrapping, I put a handful of pebbles, the kind people put in the bottom of fishtanks. Then I wrap the sheet as tightly as possible around the ribcage.
The objective is to crush the breasts as tightly as you can into the chest. For someone with larger mammaries, this can become quite painful even without the additives. If I could do it I'd put thumbtacks or marbles in there, that would hurt more, but I can't do it to myself.
For added discomfort perhaps you could position the sub on her belly once the breasts are wrapped, and have the sub grab the ankles with her hands. The sub's full body weight will come to rest on the bound breasts, and the sensation will be excruciating. Don't do it if your sub has implants, though, you wouldn't want them to start leaking or something.