You may have to mount other partitions to see the /boot directories of other installed distros. Once you've done that, you should be able to see all installed files. There should be no impediment to their visibility once the accordant filesystems/partitions are mounted.
To see all of the existing partitions, run, as root:
This will show you all of the disks and all of their partitions. I like to use mountpoints that reflect the names of the devices & partitions, such as /mnt/hda1, /mnt/hda2... /mnt/hdc1, /mnt/hdc2... etc. Then simply mount each one at the respective mountpoint (ignoring those that are already mounted for the currently running distro).
mount /dev/hdc1 /mnt/hdc1
If some partitions are formated with different filesystem types, you may have to specify that in the mount
argument list with the -t option.
Once you've mounted everything, it should take only a little browsing to find their /boot directories. Once you've found those, you can see what boot parameters are used, and transcribe them to the main grub bootloader config file.