As a professional chaser for a top tier class 9 competitor : Both Ft & RR get hammered equally if you smack rocks! Trust me on this as I've changed more flats in the last year... (PS it's the same in any class and we raced a lot of them)
You want to add cost to class 9, allow aluminum wheels. Trust me on this, as I started and sold a wheel company and learned a ton of the ins/outs. Cast wheels crack and spun wheels bend, it's a fact of life. Centerlines were only designed as an 1100lb load wheel and without higher tire pressures they will not hold up and same with a cast wheel. Steel wheels bend and repairing or making them roundish a lot easier and cheaper... We ran 17-20 in the rear and 14-17PSI front in class 9 for years and unless we made an error or ran out of talent, it wasn't the tire or wheels fault it went flat. To us higher tire pressures in class 9 were like over inflated basketballs.
Those Summit wheels look like the old wheels Bio and Richardson as well as a few others used to run back in the day.
I was at custom & commercial 2 weeks ago picking up a set of wheels for a new class 9 am building and got the same info about there not being anymore 4.5 rims made. The class 9 rules from M.O.R.E are as followed
Wheels and Tires
Must use a 15” steel rim, 5” wide maximum with any standard offset. Any pneumatic tire and size allowed up to a 33” tall and 10” wide as mounted on 5” x 15” rim with 18lbs of air.
It does not say anything about using any set combination for fronts or rear. So using the 15x5” up front should be ok just like it was ok to use them on the rear. If you have trouble fitting the wheels due to backspacing Ray at custom and commercial said he could weld the face on backwards to give the wheel some more backspacing.