There's still a long way to go...
I've thought (a long time ago, before I knew better) that I could use an orbital sander for lapping
But now that I think about it, if you could hook up a block to it, that might get you somewhere. The problem of course is that the sander is not likely to stay flat, and I mean "not at all"!
Like Bill said, circular motions will result in the block tilting.
So the linear motions are definitely the way to go.
But your first step is getting to be able to measure it.