Every trade who has been up in our roof (electricians, air con installers, sound system and speaker installers ) have all come out looking half dead and that is on both hot days and milder days. I don't think it's a place you would ever be comfortable in.
You won't find me up in the roof in the heat of the day, but in the evening I've been up in roofs changing down lights, rewiring lighting circuits, etc and it's not that bad. My parents have heaps of junk stored up in their roof space and don't have issues heat wise, you just avoid doing it during the day....
My main concern would be what load bearing capacity the steel frame has. I am not sure how kindly a roll formed light gauge steel would go compared to a timber frame at holding a lot of junk in the roof.
Personally I'd be worried about either deforming the roll formed sections or excessive deflection cracking plaster, etc if you put anything with any weight to it up in the ceiling. Happy to hear from anyone that knows any better though....