Is it OK to run a signal junction at a "do not use" pin on a solid state relay DIP package?
Unless you can get the manufacturer to make a statement on the subject, I'd assume the worst and not go near it. It may well be dead, but it could just as easily be connected to ground or something.

If you're cutting traces and running wires anyway, is there a reason you couldn't just cut the pin off of the DIP before installation? That would let you use the existing point on the board as a junction without having to worry about what's in the DIP package.