If the user has changed PORTAGE_OVERLAY (maybe /usr/portage is readonly over nfs) then layman should use it instead of /usr/portage/local.
Hi Chris! layman is capable of installing several overlays. All of those will be installed into one base directory. layman would have a hard time identifying such a base directory from the users original PORTAGE_OVERLAY setting in case there are several entries present. Did you try to change the "storage" variable in the /etc/layman.cfg file? This variable determines the base directory that will be used by layman to store the overlays.
Yes, I discovered 'storage', just thought it would be nice if the defaults worked on my setup ;-) Never mind, you're right, multiple entries in PORTAGE_OVERLAY stops that idea.