An old hanging system might work, if you don't mind wires hanging from the
beams. A lot of old estates and museums use an eye hook screwed into the
upper wall beam or cornice moulding, just below the ceiling level. From
this would hang a long wire (or two) heavy enough to support the picture
and frame. You can paint the wire to match the wall, but it still might be
too obvious...
But at least you can use the same beam and just move the eye hooks and
wires when you change.

Exhibits Director
Nevada State Museum & Historical Society
700 Twin Lakes Drive, Las Vegas NV 89107
702-486-5205