You can lightly 'score' the back side of the pieces of veneer and that will allow them to bend more easily around the wrist shape of the stock (and other features too).

Don't go too deep or the score mark(s) will show on the outside when you start trimming it back down.
The closer the score marks are together the closer arc the venier can bend w/o breaking.
Go both ways (criss cross) and the stuff can cover some convex surfaces but not nearly as well as the commercial 3D-Veneer thats made already cross cut on the backside.
The commercial stuff is hard to find and $$.

I wouldn't even use the 300* activated glue that's on the pre-glue on the veneer. Just fit the pieces and use an epoxy or even a simple wood glue is all.
The epoxy might be better as there usually some slight voids to fill in under the wrap around method on an old surface such as a gun stock.
23weyth6Pre-glued veneer does work well on perfectly flat surfaces prepared as such.
If there's a void under the thin veneer (unsupported) there's a very good chance of pushing through it in finishing or even later handling. Then you have another 'fix-it' project within your fix-it project.

Using epoxy to build the surface works well. Then you have to seal it. plain Shellac works . Then use Earth colors to fake the wood grain and match it in. Then 'fix' your art work before the final finish over all. I use a spray enamel very light 'fix' coat over the colored up fake wood. Carefully sand that a bit. Then finish the whole stock.