Well, the man page says otherwise, and I'd hate to rely on undocumented safety buried in the standard library.

Edit: upon re-reading the man page, this bit caught my eye:

"If you need to use these frameworks in the child process, you must exec. In this situation it is reasonable to exec yourself."

So that would be one (highly painful) way to handle this safely. Fork, then self-exec, passing in arguments that tell the newly execed process what you really want to run. The new process can then do the /dev/fd listing in peace, then call exec again. Eww.