I just had a quick look, if you edit the "Force URL to Style" plugin and replace
Code:
if ($vbulletin->options['ddds_36_enabled'] == 1)
with
Code:
if (($vbulletin->options['ddds_36_enabled'] == 1) && (!$foruminfo['styleid']))
it should not override a style explicitly defined in forum properties. This was tested so minimally that I may as well have not tested at all, so no promises. Let us know if it work.s