Marc:
> I have multiple releases installed on separate LVM partitions. They
> frequently are using the same kernel, so I was thinking that it would be
> convenient to have a separate /boot partition and a separate
> /lib/modules partition that could be shared between them.
...

I long time woundered why the modules isn't stored together with the
kernel (which currently is on /boot).

Why not have /boot/modules and mount the boot partition at your bootup ?
You can do:
cd /
ln -s boot/modules lib/modules
in your different distrubution roots.

/boot can be ext2 or 4 and mounted ro to make things simple and ext2/4
be included in the kernel image.

Regards,
/Karl Hammar