My two cents:
I grow some plants
in 1/2 wine barrels. I found that the barrels will last longer when there are drainage holes at the bottom of the barrels and the barrels are used upright. Over time the woods will dry/shrink or misshapen and will pull away from the metal bands.
On it's side, the barrel will probably be filled less than 3/4 full, so there will less soil to push the wood staves against the metal band. There's a chance that the staves with no contact with potting soil will dry out quicker, shrink and pull away from the metal band.
That said,you can try a combination of plants
For full sun with regular watering:
Black-Eyed Susan [Rudbeckia birta]
Asparagus Fern [Asparagus setaceus]
For part shade with regular watering:
Leatherleaf Fern [Rumobra adiantiformis]
Lilly-of-the Valley [Convallaria majalis]
Variegated Ivy [Hedera helix 'Glacier']
See Longy's post about potting soil.