Thank you very much for those two very useful replies.
Yes, putting the simple line
into the OS X section of /boot/grub/grub.cfg allowed me to boot OSX86 through Grub2 at the desired resolution.
Although I haven't tried it yet, the second reply also shows how to chainload OS X from Grub2 (which is not as simple as chainloading Windows). So, assuming that will work for me, it should mean I can boot OSX86 through Darwin from Grub2.
As Darwin recognizes com.apple.Boot.plist while Grub2 doesn't, and as I've included the kernel flag "Graphics Mode"="1680x1050x32" in the Boot.plist file, this should also allow booting to the desired resolution from Grub2.