I read about this method for flattening large slabs in a publication (Woodwork I think) a year or two ago. Unfortunately, I couldn’t find the article in order to reference it. As I work with irregular slabs, too large for my machines, I expect it to be a reasonable and effective method for flattening large slabs, in this case, eastern red cedar.
The router is mounted to a 4′ sled which rides across two parallel fences (jointed 2×4′s smoothed and waxed). The router has an 1-1/4″ straight cut bit, and mounted on the sled can cover the entire face of the slab, removing about 1/32″ of material per pass.
a couple of turned walnut handles help to steer
the whole endeavor sits on my table saw