We tried that with a #4 Camalot and a chunk of 2x4 once and it will hold, but you need both hands to place it. I think there's a pic of this technique in Yosemite Climber, on Excalibur?
it works more or less the same as nailing a bong in stacked against a chunk of wood will, in that it will hold you to rest on, but who the hell knows how strong it is if you fall