Your method for adding the handler to the RDB sounds correct. But the missing piece might be to match the dostype of the partition to the dummy-handler. What that value is, I'm not sure, but I think HDToolbox displays it when you add a filesystem to the RDB. When you set the partition type to custom, it should allow you to enter the dostype as a hex value.
Another possible option: the version of HDToolbox included with OS3.x (it works under OS2.x) actually allows you to directly set a partition type to dostype UNI\01 and make it bootable. Then you can select that partition in the early startup menu (or give it a high boot priority) to boot from it. I ended up partitioning my drive for Amix entirely from AmigaOS. Whether this will still work from the GVP controller is anyone's guess, but I suspect it will be enough to get the kernel booted, and if the kernel has the GVP driver it should be able to take things from there.
