I've given this a little more thought, and I might have been too hasty, though technically correct. config net might just work, but I don't have a system on which to test it.
What I realized is that the Pix config given by "write term" or "write net" very well might contain all necessary statements, such that if you issue "write erase" you'll get your config back.
An issue with routers is that the config file doesn't contain many default statements, little things like "no shut" for the interfaces. Cutting and pasting or (if it was possible) config net wouldn't actually put the whole configuration on the router. With a Pix it might just work. Since you're erasing the configuration anyway, try it and see.