I don't see flexibility being much of an issue here.  In fact, everything I have read indicates that the less flexible the adhesive the better for veneer. 

BTW, for future reference (it didn't occur to me to send this in response to your earlier questions), you can make your own bow cauls pretty simply with a hand plane.  Before I started using hand planes, I actually did this with a jointer too.  This link has simple instructions on how to do it.  I have made a few of them over the years and they are great anytime you need to clamp over wide areas.  You can use them to clamp down veneer but I mostly use them help keep panel glue ups flat during clamping  You can make them for one time use out of 2-by stock.    I usually line them with packing tape and apply paste wax to prevent squeeze out from gluing them to my workpiece.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.