I'll take a stab at it...but keep in mind that this information is worth what your paying for it...so all the standard disclaimers apply...
What I'd do is before drywalling, fasten nailers between the studs at the correct locations. Start with angle iron pieces, one for each tread that is 10 inches long with a 2" and 3" leq and 1/4" thick. Lag the 2" leg to the nailers, with the 3" leg making a ledge for the step. This leg sticks through the drywall.. You could either set the tread on the shelf or bump the angle up and cut a kerf in the end of your tread and fasten the tread from the bottom.
This image shows the rough idea...of course you'd want to use strong material for the treads.
Alternatively if you don't want drywall next to the treads, you can always use 1/2" plywood, tape the seams like drywall, skim coat, sand, texture just like drywall, and way stronger. 90% of the people wouldn't ever know that it wasn't drywall.
Hope that gets you started...
I'm new to the forum and hope this is in the right place. I'm wondering where I can find heavy duty angle brackets like the ones you describe here? Or will have have to have them custom made?