I have not tried this, as I don't have a dressmaker's dummy, but here are my thoughts on the matter, it's something I've been considering myself.
If possible, set your dummy to a little less than your corseted dimensions. Construct a firm cover - perhaps using coutil - that is exactly your corseted dimensions and desired shape and apply it to your dummy then, using wadding pad the voids firmly. Next, construct a looser cover (muslin, jersey?) that is exactly your natural measurements and shape, and again apply and pad.
As for using jelly to pad the belly,I would look into how pregnancy belly simulators are constructed, and make one in the size and shape of your belly, (start with the two different covers to determine the shape) you can then insert the jelly packets into this belly.
I think that this should work, in theory anyway.
This is obviously a lot of work to do for a client, but for yourself, I think it would be worth it.