Is there a way to have separate filesystems combined in mount points to look like a single high-level filesystem for mapping a single drive letter?

We previously split up our shared business groups filesystems because they became very big. This was to reduce the outage if a host server failed and to divide and distribute the work of data back-up clients and anti-virus utilities. A consequence was a complicated set of eDir objects (DirMaps, Aliases, profile log-in scripts, confusion documentation, etc.) to support a user from one business group getting an additional drive mapping to a different business group's shared files. Going to SLES/OES we want to be free to keep the filesystems separate but now have a way of presenting all the business group shares back into a single filesystem. Ideally, everyone gets the same drive mapping to the base of the organization shared folders and access is controlled simply by group memberships (which would be natural for a smaller single filesystem).