Temperatures in a mash tun can get up to 175+ degrees during a mash out. Most PVC is only pressure rated up to 140 degrees. That having been said, There is not really any pressure on the pipe.
According to wikipedia, the glass temperature of PVC is ~180 F and the melting point is 212 F.
I think you could get by with it in the short term but I would be careful about what temperature you put stuff in there. For instance, when I mash out, I usually drain water, heat it and add it back in at boiling temperatures. I don't think this would be wise with your set up.
You might consider replacing your manifold with a stainless steel braid. It's easy to find and cheap to make.