If you're positive that you have a BCM4312, then the fact that
bwi(4) doesn't attach is indicative of a rev 2 device.
Because the rev 1 chipset
is supported, it is thusly identified properly.
A supported card would appear as:
bwi0 at pci? dev ? function 0 "Broadcom BCM4312" rev 0x01: apic ? int ?? (irq ??), address 00:00:00:00:00:00
While an unsupported card would be clearly identified as such:
"Broadcom BCM4312" rev 0x02 at pci? dev ? function 0 not configured
I hope that helps.. as for NDISWRAPER, the answer is no.. that is not available for OpenBSD.