I started working on this for a while and then dropped it.
http://www.roundup-tracker.org/docs/customizing.html#stop-nosy-messages-going-to-people-on-vacation
The annoying thing is that you have to copy and paste some core roundup code to
make the email backend work.
Note that we have the user interface part of it written (which is easy), it's
just making the nosy reactor be more selective about who it sends email to.