Wow Haven't seen that aluminum stuff before. That's a cool option. I think I would take a real hard look at fiberglass before going with wood. A steel structure with fiberglass over the top would be really durable. I wouldn't trust Luan or hardboard to be strong enough. While you describe the structure as hopefully not taking a lot of abuse... it sounds like there is a possibility of it getting knocked around a bit. 1/4" would be strong enough to survive a fairly substantial accidental jolt.
I think the lowest budget approach would be to build a steel tube frame and face it with 1/4", take hardboard and cut it into squares . Attach the hard board squares to the 1/4" ply to create some depth to your concrete blocks. Texture with Van's V.S.S.D. (Van could he skip the hard board squares and texture the 1/4" directly? Sounds a little more tricky to get it right).
Should be strong and not terribly heavy. Remember that "light weight" and "strong" are words that can only be combined through the application of a lot of money.
I'm having trouble tracking down fiberglass panels, any thoughts? How much more durable do you think 1/4 ply will be than Luan? I am intrigued by the texturing idea, but texture is really secondary, getting the walls to be solid and stable is far more of a concern to the folks I am working with than the precise look. They'd probably settle for grey walls with black lines for the stone as long as the walls were sturdy.