Maybe the info for grub2 in
ftp://ftp.eu.openbsd.org/pub/OpenBSD.../INSTALL.linux helps ....
Code:
Booting with GRUB2
-----------------
Here is a sample configuration for OpenBSD and Windows 7
menuentry "OpenBSD" {
set root=(hd0,3,a)
chainloader +1
}
menuentry "Windows 7" {
insmod ntfs
set root=(hd0,1)
chainloader +1
}