I would just like to leave electronic evidence that Mantis Bug Tracker works on the top of ONPP stack (OpenBSD, Nginx, PostgreSQL, PHP) without any problems. It works in the chroot of course. The only "tricky" part is manually creating config_inc.php from config_inc.php.sample file as well making sure that permissions are correct since Mantis is no longer in ports.
By the way if it was up to me I would use fossil and proxy fossil server with Nginx. Works like a charm. However my co-workers wanted Mantis due to its integration with CVS and Subversion.