I am as surprised as you that the OBSD router/firewall somehow prevents access to the peer nodes.
If you have a
block log all statement, all blocked connection attempts will show up on the
pflog0 device. You can run tcpdump on
pflog0 to watch these blocked packets. See
plfog(4)
You could configure a spare box as a bridge as per
http://www.openbsd.dk/faq/faq6.html#Bridge. This allows you to watch the traffic with tcpdump without disturbing anything.
Place this box between a peer node and the switch and you can watch/dump the traffic transparently, in other words without affecting the traffic in any way.