A SBR is still considered a "firearm" and thus falls under 922. You cannot have multiple evil features on a post ban gun, even if you make it a SBR. I'm positive of this.
The only way that you could put evil features on a post ban gun would be to turn it into a machinegun (the gun would then be considered a "machinegun", not a "firearm" - I know, it's stupid, but I didn't write the rules) -- and the only way that you could turn a post ban gun into a machinegun would be by dropping a registered DIAS into a post ban AR. Or by being a Class 2 manufacturer and then you could drill the holes. But then you would have a post-dealer sample - transferable only to LE or military.
BTW, the tax on a SBR is $200, not $5.
So, net/net, no you cannot have evil features on a post ban SBR.
Sorry...
danbrew :->