I've shellacked cork (well, fake cork) tape and it held up surprisingly well; I didn't notice any cracking from the flex. I suppose it should be noted that this was on a pair of porteur style bars, with just the grips covered, rather than a road bar, just in case this makes any difference.

Also, if you shellac, the twine is completely superfluous, the shellac will hold on the tape quite well on its own.