Well, the 15 disks limit was done because it is said that some BIOS would have a problem - and BIOS disk 0x90 was the same as BIOS disk 0x80...
But as long as your vmlinuz/initrd pair is located in one of the first 15 disks Gujin should still work.
Moreover, I assume that you start your system with some disk in "power on standby mode" - or else you have a massive power supply; those disks will not contain the /boot partition.
Gujin has been tested with disks in "power on standby mode".
With such a system, if your main MBR is corrupted and you try to boot from a USB key, and that key is mapped by the BIOS as disk 0x80 (shifting all other disks numbers by one), your preconfigured mapping in either Grub or LILO will be wrong (trying to load from the wrong disk).
Increasing the default number of disk supported in Gujin is a single "#define NB_DISK" change, maybe I'll increase the default.
> your description of the many-OS system can still be done with very few drives
One drive is enough, but the complexity to manage the six Grub configurations in six partitions is a nightmare...