I've been using the ROOT=/... function of emerge to crosscompile my own livecd. Nothing new. The difference is that I'm using unionfs to build in modules.

mount -t unionfs -o dirs=/modules/apache2:/livecd=ro none /modules/apache2

I do all this in a chroot from the actual system (on the hd) that I'm installing to (ROOT=/liecd). The "Gentoo installer" is really nothing much more than the stage3 tar I'm using for emerge.
mount /dev/hdb2 /mnt/hdb2
mount -t proc none /mnt/hdb2/...gentooinstaller/proc etc
mount --bind / /mnt/hdb2...gentooinstaller/livecd
chroot gentooinstaller
ROOT=/livecd emerge stuff

But now I want to test this module by somehow merging /mnt/hdb2.../modules/apach2 with /. So far I havn't had much luck.

I know it'll work if I copied everything over. But that's not the point. I want to be able to load/unload "modules" while in the livecd either on the CD or into RAM. I'm sure it wouldn't be that hard to set up slax to boot different configuratiosn of the modele (for instance one w/apache2 and one w/o via cp -Pr both w/ or w/o toram). That's a boot option though that cant be "reversed" once in the session.

So how can you do something to the equivilant of:
mount -t unionfs -o dirs=.../modules/apache2:/ none / ? (which btw doesn't work)

I got it...I'm supprised out of all the people who post on these forums, nobody knew.

Instead of mount you use unionctl, eg, unionctl / --add module