Hmm i don't quite sure about advanced stuff on freebsd cause i am new myself, so i don't understand about your mentioned userspace library .wish add libnetfilter_queue feature.
libnetfilter_queue is a userspace library providing an API to packets that have been queued by the kernel packet filter. It is is part of a system that deprecates the old ip_queue / libipq mechanism.
libnetfilter_queue has been previously known as libnfnetlink_queue.
so libnetfilter_queue will work for snort?