Having an independent process/thread updating the servers list is exactly the
idea (but it would be able to handle more complex server entries like '8/ssh -p
2222 host.net' too).
However, as others pointed out, perhaps that daemon should not be part of GNU
Parallel's main code. It should suffice to have the ability to change the
server list at runtime. Then, the users could run whatever they want to update
the list. An additional benefit of this approach is that the user has more
flexibility/control regarding the updating method (maybe he or she prefers a
serial script or that certain servers be checked more frequently than others).