yes, /etc/network/interfaces should not be altered manually. It is built fom UCR variables (
ucr search --brief eth0 and
ucr search --brief gateway) and the templates (in /etc/univention/templates/files/etc/network/interfaces.d/).
It appears that there is no direct support for post-up and pre-down but the ipv4(ucr, interfaces) function in /etc/univention/templates/files/etc/network/interfaces.d/10-default has
for value in iface.options:
print '\t%s' % (value,)
This might enable you to define the additonal lines.
ucr set interfaces/eth0/options/1='post-up route add GATEWAY_VM dev eth0'
ucr set interfaces/eth0/options/2='post-up route add default gw GATEWAY_VM'