The slab/foundation is the sill. Heavy timber sills are for rock foundations and such. Stick builders need the 2x plate to fasten the studs to. Depending on the size of the building you may only need 4 point where the frame contacts the foundation. Now, you will need something to fasten the wall system to. Did I read straw as a possibility? SIPs would use a plate tacked to the slab which the panels are routed for.
