Aluminum...
I wouldn't see why not, it may not have the thermal mass as same thickness steel, but insulation would take care of that if heat loss became an issue. If these were made for ballast, they are probably pretty thick already. If you have a mig or tig welder (or know one) that wouldn't be an issue either. Corrosion wouldn't be a problem, and aluminum does accept paint with minimal surface preparation, and wouldn't require any more upkeep that a steel (maybe less because of the self arresting aluminum oxide layer preventing additional corrosion).
My guess is that most q-er's don't use aluminum (or stainless) because steel is a heck of a lot cheaper. If we all could afford stainless, we'd all use it.
Most of the standard gasser grills are cast aluminum, so a metal toxicity health issue obviously isn't a concern.
So what did I miss, why wouldn't you use aluminum?
If I was anywhere close, I would hesitate to go pick these up for the same purpose.
Maybe Norcoredneck could chime in here to with an opinion.
RMR
