First I have heard of it Richo, and I do not believe it to be true.
No doubt there are a number of signed sets out there, in fact the first set of these I ever owned (which I still have) is signed by Miller. It is #199/500, and this set is #473.....so clearly there does not seem to be a block of sequential set numbers (ie sets #'d 401-500) that were signed.
The other reason why I doubt this is that I also bought Alan Connolly's "player" version set a couple of years ago (each player in the set was given a complimentary set to keep stamped "Player" of 500 on the back of each card in silver foil - by the way Alan Connolly is card #53/60 in the set) and with it came a letter that was sent to Connolly (and all the players in the set) along with the album. Read the 3rd to last paragraph:
There is obviously nothing stopping someone (ie Steve Wang) buying 100 of these sets upon release and knowing someone to get Miller to sign them unofficially, however Miller certainly did not officially sign 100 of the 500. Those he signed were done after the release.