You can give bootdevice option to boot.img in OF. For example "boot boot.img rd bootdevice=dh2". And remember to use different boot.img for MorphOS 1.x and 2.x... rename them like you want in your boot image partition and give correct one depending which partition you want to boot.
If you want to keep it in more permanent use, you can make bootmenu with bootcreator for example. I have bootable partitions for 1.4.5, 2.x and backup system with it on my Peg1...
Afair on my PegI's I use the "bootdevice=dhx" for the MorphOS 2.x boot.img and then I have the MorphOS 1.4.5 partition set with a higher boot priority than the MorphOS 2.x one as the 1.4.5 boot.img doesn't accept the bootdevice= flag.
Hmm.. 1.4.5 boot.img surely accepts the bootdevice option? There is my bit outdateish and messy boot script for Peg1, but you'll get the pic... bootdevice takes the partition's device name as an argument.
Btw. Aminet link seemed to be for BootCreator 1.1, but there's newer one somewhere too.. I think it's needed to get automatic timeout for Peg1's menus etc..
Well after all this bootdevice flag worked for me and using the ls command I found that the OF uses the exact names that I have given to each partition using the HDConfig tool. Great!