I had to do that with a mag tape once, but it was just a case of monkeying around with JCL parameters and DDing a few hundred bytes into files one by one until

I had to do that with a mag tape once, but it was just a case of monkeying around with JCL parameters and DDing a few hundred bytes into files one by one until I figured out where the surviving data was, then just use the right JCL to make it transparent to the program. I couldn't figure out why I couldn't get BLP to work, though (NL did the trick).