trueing an irregular slab

I have coffee table sized slabs with irregular surfaces from tree buttresses. By erecting four threaded rods on my bench and using them as supports for two angle iron parallel base rails running lengthwise along the bench, I can run my router with a square straight bit on another pair of angle iron rails resting at right angles to the fixed ones.This setup forms parallel planes of varying heights on each slab as I raise the base on the threaded rods and move the second pair along the base rails. It creates an interesting match of plane surfaces- up to three on a decent sized slab - with the rough natural rises stepping up from the lowest plane surface to the next. Basically I'm using the router as a very narrow bladed plane, but am able to keep each plane surface on the table true - provided of course the timber is fixed for the whole operation. The work takes time but is the only way I can work out to make a slab of solid timber for a not too shabby table top.