Finally settled for this option:
Pooling only software option: http://romanrm.ru/en/mhddfs
It allows you to have two disks with separate (and complete) filesystems, and make those appear as a single tree from the LAN. Directories with the same name will be merged when listed. You start with one drive and when its full, it will create the file on the other drive (if it has free space). If you disconnect a drive, it stops showing it's content.
Other options where:
Alternatives to LVM and JBOD (check "OS/software"):
http://blog.superuser.com/2011/09/14/building-a-nas-server-2/
EDIT: Other software option: http://snapraid.sourceforge.net/
JBOD and LVM volumes fail completely in case of a disk failure:
http://forum.elitebastards.com/viewtopic.php?f=6&t=16838
https://serverfault.com/questions/338517/what-happens-when-a-disk-fails-in-lvm
EDIT2:
As suggested by @lelouch, mergerfs sounds like a better mantained and updated option to mhddfs. On defense of mhddfs, I have been using it with no mayor issues for the last 4 years (apart that one or two times where it dropped the mountpoint for no reason, but after rebooting everything was in place again).