Well it's not rocket science to Boot MorphOS (PuP) from everywhere
Since MorphOS boots by just using a Bootloader command you can boot it from every partition you want (even from a loaded OS 3.9). You just issue the bootloader command and a reboot occurs. Based on the bootdevice that you declare on the bootloader, your MorphOS partition will load just fine.
So it was really easy to just make a small Startup for ABS to just load the Bootlader of Morphos and be done with
I suspect you can also Boot OS 4.1 from ABS the same way, although you'd need to select the OS4.1 partition from OS4.1's kickstart Early Startup Menu once ABS loads the bootlader and reboot occures. Unless you start messing around with Thomas's excellent ChangeBootPri command that is kinda more hardcore but more effective solution inside ABS's startups.