Hey Paul
Congrats on being finished with the rad tx.
I would suggest water at first, just to see how you're handling things. While all of us have different reactions partially because of the various locations of our zapping, you may or may not have trouble swallowing. Just take it easy at first, then work your way up the viscosity scale.
Now on to the PEG. I put anything and everything down it that would flow. Gatorade was definitely on my list, as well as V8 and many other water down blended recipes.
Kevin