This mis-feature is incredibly annoying.
Has a new service pack been released in the few months? You could start with that.
You could also try to set the DXF export ratio using a simple macro, maybe it can be set that way? This page lists all the available options:
Of course, in keeping with expectations, I currently cannot download service packs:
"An error occurred while processing your request.
Will poke around the API. I don't get why you can't just check the grey'ed out 1:1 box.