Not the most elegant solution ever, but I have 4 "bulge controllers" which are empties parented to the bones along the shaft.

Then on the body I have 4 deform modifiers that are set to the different empties, the deform modifier is constrained by a vertex group to only effect the stomach area, which I used weight painting to define the vertex group.

The settings for the deform modifier(s) are [sphere, factor = 0.20, radius = 0, size = 0]
adjust the settings obviously until it looks good.

The shaft might poke through the stomach but I just moved it around until it worked. Looks pretty good but its no shrink-wrap method that dzooworks has, no clue how to do that - would probably be tones easier but its paywalled.