I still have an old Linaro image on my card (Ubuntu 11.05), and I
did quite a bit with it when I was messing with it.
It looks like I was thinking of /etc/network/if-up.d, but I don't have
anything related to DHCP in there. I'll poke around when I'm
less tired to see if I can find it.
IIRC, either /etc/dhcp/dhclient.conf or the command line sets a PID
filename that is then used to control its behavior. If you want to know
exactly how it was invoked, one method is to examine /proc/$PID/cmdline
and perhaps grep for fragments of the arguments. I think that's how I
found it last time.
As for the rogue client being cured on reboot, I don't think that the
network upstart script actually kills dhclient via the pidfile. The config
change should be sufficient now that you've rebooted.
Sorry I was less help than I hoped to be.