Well maybe it would be possible to modify the early startup menu itself to allow to boot into different "Kickstarts", eg. MorphOS, OS4, or OS 3.x.
For example on Macs you hold down the Option key at power-on and you can select to boot from different partitions or the CD drive. Maybe you could select different "areas" of the Deneb flash, if it was segmented.
But the goal was to be able to put the OS4 Kickstart files into the Deneb Flash to make it resident at boot. Such capability already is there as you pointed out for MorphOS and OS 3.x kickstart updates.
If it was possible to also allow switching between resident MorphOS or OS4 or OS 3.x BB2 updates, that would be superb.
But I am ready to kick off a donation/bounty if we get some initial interest.