Chainloading GRUB from Win7
 

Yes, you read right.
Windows 7 has steadfastly refused all attempts to make it boot sucessfully by chainloading from GRUB to it, so now I'm going for the opposite way.

With Arch (finally) installed sucessfully, with it's own /boot and root partitions, I've used EasyBCD to create an entry in Windows' bootloader that points to Arch's /boot partition.

And nothing happens. Windows works fine, but it appears incapable of loading Arch from the /boot partition.

This might be because GRUB was installed to the MBR, found incapable of loading 7, and then re-overwritten with the Windows loader, so I'm guessing I may well need to reinstall GRUB, this time to the /boot partition (sda5) - except I haven't figured out how to do that with the pre-existing install.

In short: Arch is installed on sda6, with /boot on sda5. Entry created in Windows pointing to sda5 to boot, but nothing happens.

Yep, that's all you need to do - the BCD entry should find it o.k.

Last I installed Arch it used classic grub (i.e. not grub2). As root from a terminal this should work

Right now, the problem there is getting into Arch itself. I know the install cds can be used as rescue as well, but from there on my knowledge sort of fails me.
If memory serves, mounting the root and boot partitions in a temporary folder and using chroot should work. Not entirely sure if that's all I need to do to make it work though.

You do know that windows 7 uses two partitions. One to boot from and one for the windows system. Sometimes three if there is a recovery partition. This throws some people off because they will try to chainload to the partition that the main system is on instead of the boot partition. Never mind I miss read your first post.

That's probably what threw me off. I've managed to get it to chainload this way sucessfully though, which is enough to suit me for now. If it gives trouble later, I'll figure this out.