Spot, I doubt it is going to be as late as 3/13. That is when all YOM plates already issued expire. For example, if I registered my plates last year, they prorate the fee so that they expire 3/13. Same goes if I registered two years ago- prorated to expire 3/13.
So, in response to your question about 3/13 being that latest you can register, I think the answer is "no". I'm only running on gut instinct, but it would seem odd to me that someone could walk in and register their plates 12/12, pay for only 4 months of use, and then renew in 3/13.
The original post in this thread mentioned problems after next year. I place my bets on no more YOM registrations starting 3/12. The key issue comes down to when Texas turns over this aspect over to myplates.com.
Just my $0.02.
Bill