Actually if you sponsor it there might be a configuration that works and does what you ask for!
FreeBSD has vimage which gives you different net stack instances on the same hardware you can combine that with jails each running a fpfSense instance on each virtual stack and that gives you what you want and since nokia and juniper started from freebsd they can support such feature, and this is a wild guess, just cause of this feature in freebsd.
vimage is only present on RELENG_7, so you have to wait till then or try to experiment yourself.
Regards.