If you were talking Jet Hot specifically, then I couldn't comment. BUT, we have HPC over here and you can weld over their coating. I had some other pipes porcelain coated through another company, and they couldn't be welded.
It does discolour the surrounding area, but you definitely can weld the HPC. I was talking to them two days ago, because I need to weld in some exhaust temp bungs for datalogging purposes.
Here's an old photo of my pipes. Does the finish look the same as HPC USA?
