If anyone is interested, I've submitted a patch to Rob Brown (maintainer).

The patch is available at: https://ulppd.svn.sourceforge.net/svnroot/ulppd/trunk/errata/Net-Server.sock_owner.patch
(can't link it; filename must contain characters the link parser can't handle)

Just pass the options sock_owner and/or sock_group to Net::Server to change the owner and group (respectively) of unix sockets. If either or both options are omitted, the previous behavior is unchanged. Hope someone finds this useful.