I'm not familiar with this particular product line, but if the data sheet says that pin is required to be held high, and has been shorted low, then you can't use the ISP. I suspect there might be some way to use an external programmer to program it (the data sheet will tell you if there is), but I would agree with Patrick de Groote. How can you trust anything else about that part? I suppose that if all (that's every line and subroutine) works perfectly anyway you will have that, but what will you do if any of your code doesn't work? I would hate to spend a lot of time debugging code only to find out that the problem was a fried register bit. And as a final thought, is the part really more expensive than an external programmer?