Cool mod. What about threaded braces which could tie the back of the lights to the bumper grill slots and adjust the brace compression with the threads. It wouldn't tie the light bar to the subframe, but it'd at least give you a couple more points of contacts to the light housings.
It's neat that you rallycross the TC. I've noticed these vans corner pretty well stock and manage to have some fun up in the twisties whenever the opportunity arises! Do you have a manual conversion? If not, how does the auto-shifting hold up in your races?