My only concern, and it may be irrelevant if the racks are overbuilt, is that making them out of aluminum might prevent you from being able to readily find a repair station in remote regions of the US and/or 3rd-world countries in the event they became damaged. Steel welding, and broken racks from adventure riding, are universal. 'Just reading a few ride reports easily confirms this.
But finding someone to weld alloys in remote regions, well that's a potential problem.
'Not knocking the desire to keep things light, only concerned about the possible problems having a irreparable rack out in the field could cause...