I realize that since NFS is not block-level, LVM can't be used directly.

However: is there a way to combine multiple NFS exports (from, say, 3 servers) into one mount point on a different server?

Specifically, I'd like to be able to do this on RHEL 4 (or 5, and re-export the combined mount to my RHEL 4 server).

expansion
The reason I pegged lvm is that I want a bunch of exported mounts (servera:/mnt/export, serverb:/mnt/export, serverc:/mnt/export, etc) to all mount at /mnt/space so that my /mnt/space on this server (serverx) as one large filesystem.

Yes, I know that re-exporting is generally a Bad Thing™ but thought it might work, if there was a way to accomplish this on a newer release as opposed to an older one

From reading the unionfs docs, it appears that I can't use it over a remote connection - have I misread it? More accurately, since Union FS merges the contents of multiple branches, but makes them appear as one, it doesn't seem to go in reverse:
I'm trying to mount a bunch of NFS points in a merged fashion, then write to them - not caring where data goes, a la LVM .