Actually I got it all going, and the MacBookPro worked as planned, but the bootp server offered a filename of pxeboot and the silly Intel 10/100 card decided to fail due to missing files on the tftp server. Many hours later and I figured I should use a network sniffer instead of trying to turn on tftpd logging.
A packet dump showed \377 or 0xff being appended to the filename by the boot client (intel Nic on remote server) when using tftp to request the file (making filename "pxeboot" look like "pxeboot\377" in wireshark's view of the packet, as discussed here)
Since the Mac bootpd.plist file has no option for "filename" documented and since there appears to be a bug in the bootpd implementation with respect to supplying dhcp_options as per RFC2132 (Paragraphs 9.4 and 9.5) it was just too time consuming to stuff around.
If I get a full working implementation of dhcpd/tftp/pxe using the Mac - and actually working independent of the intel nic problem on the server, I'll come back and post the info here. Don't hold your breath though. I rebuilt the OpenBSD server using bsd.rd (Ramdisk installation).
The idea is that the DHCP server will hand out an address, and the TFTP server's IP address as well. Also, the DHCP server should hand out the boot filename too, then the client contacts the TFTP server and boots from it.
I use tftpd32 on my Windows box, and it works like a charm. The program even has a built-in DHCP server, so you can cross-connect to your server without the need of configuring your network gear, although I do the latter at home. All I do is plugin and boot from the NIC, and I can load any client.