Hmmm, this got me curious.
I pulled out TeeDee's crossbow and compared the string to 2 mm Dynaglide. From a visual comparison, the Dynaglide is slightly smaller in diameter. I know the crossbow string is dyneema or spectra, but I have no idea what the breaking strength is.
As far as using a whoopie? Do not really know how it would work in that application. The sudden application of a large force when the bow snaps back could be quite a bit different from the normal applications for a whoopie. Seems like you would have an increasing force as the bow is drawn, then a sudden release of the forces on release of the string and then a huge spike in force as the bow limbs snap back all the way.
Only way to tell though would be to make up a whoopie and use it. Safely of course in case something breaks.