Jim,I really don't know the answer to your question about the danger of grafting outside.
I will say that plastic is never a good idea where bright sunshine is a factor.
Now,and all the following is guesswork,I would not hesitate to try what you want to do.
Provide some shade if you can and I also think this might be a good place to use grafting wax or beeswax to seal the graft union and prevent moisture loss.Go for it.Worst case you end up with a shorter plant.
