Finally! I got the machine booting in legacy mode.
If anyone is struggling with a 4,1 or earlier Mac (creating an OS X/OpenBSD dual boot environment on a pre-exisiting OS X install), here's an executive summary of the process:
1) Prepare the hard drive from within OSX with 'Boot Camp Assistant' (this will resize osx, create a new 'Windows' partition, and change the partition table from GPT with protective MBR into GPT with hybrid MBR)
2) Reboot from within 'Boot Camp Assistant' with an OpenBSD CD in the drive (machine will boot directly into whatever bootable cd is in the drive)
3) Install OpenBSD as usual (disk is wd0, new bootcamp partion is Win95/FAT32, set type to A6 in fdisk)
4) reboot & done
Unless the setup is changed the Mac will boot henceforth in Legacy/BIOS mode. Supposedly it works like this: on powerup/reboot the firmware inspects the drive's partition table and boots in BIOS/legacy mode if it detects a classic MBR or a GPT/hybrid MBR and if a partition in that MBR has a bootable flag, otherwise it boots in EFI-mode if it finds a GPT with protective MBR or no MBR partition is flagged as bootable.
It may be advisable to have some useful tools at hand in case of an emergency, like a rescue-cd or USB-drive with rEFind, gpt fdisk (aka gdisk). Sth. like that.
Obviously the hybrid Frankenstein partitioning is not exactly a desirable setup but at least it works for the time being. Until, hopefully, one day the OpenBSD EFI bootloader may also support UGA for the graphics protocol (keeping fingers crossed!)
In honour of this happy milestone I hope my exuberance may be forgiven when I state that clearly some
JB is in order.
