Just found out what is going on here: src/dhcp-manager/nm-dhcp-dhclient-utils.c
tries to determine whether the dhcp-client-identifier is a hex string or not. The string I used was something like 1:0:ab:aa:ab which is a perfectly acceptable hex string for dhclient, still the text identifies it as an ASCII string and thus encloses it in double quotes. When I change this to 01:00:ab:aa:ab it works, NetworkManager generates a dhclient.conf without double quotes around the client identifier. So maybe the test should be changed to accommodate such cases...
