Thanks to all for your support and ideas! I finally was able to work around the issue.

I did a MorphOS 3.10 install on the HDD conntected via PCI-SIL3114, made a HFS boot partition on the HDD (dev/sda9). I rebooted into Linux, copied the contents of /dev/sda9 to my HFS boot partition on the DOM (dev/sda4). Used hfstools to 'bless' the boot partition on the DOM via

Code:
... and voilá, my DOM's HFS partition now pops up in the OF-bootselector and is usable via yaboot. Now Linux and MorphOS can happily coexist again on my G5.

So the problem is solely rooted in HConf which freezes when trying to save the changes to the DOM on <2nd.ide.device:1>. Being unable to rename the otherwise fine HFS partition to Boot: IWizard won't find a boot partition and the installation can't continue beyond this point.