1/4" luan has very little strength/stiffness on it's own. You would need a lot of support structure to use it. Luan has less strength than normal ply (unless it has changed) as the plies run the same direction instead of at 90 degrees to each other.
The spar will protect it well, but it will not add strength. Is is just a varnish, after all.
As far as being slippery...probably. You can always add some traction with sand or pumice though.