This program has been discussed on several vintage muscle car forums as well as the SEMA lobbying site (if I recall correctly) for a while now. Due to the fact that the legacy plates will be "Similar" but not identical to the real YOM plates, coupled with the fact that the legacy plates can be assigned to any car including a new 2015 model, lead to a number of opinions that the YOM was the preferred option for those year model cars eligible for YOM.
Notwithstanding the availability of legacy plates and the YOM program, I see all sorts of cars at car events with more modern plates that have been refinished to simulate the 1963-69 black with yellow letter format, even when the plate has the 7 characters configuration.
At one time several years ago there was a guy on the east coast who offered "novelty plates" containing any combination of letters and numbers you wanted that were visually quite accurate depictions of "legal" California plates. Apparently someone got grouchy and the State probably sent him a cease and desist letter as he no longer does any California novelty plates.
Technically illegal but not yet generally enforced in any dragnet sort of effort that I have seen. I expect that so long as the plate is legible and depicts the actual number assigned to the vehicle, LEO will likley focus on more nefarious crimes. Then again..........................