Are Johto Starters/Sinnoh Starters/Unova Starters shiny locked in ORAS?

Nope, they're definitely not locked. Professor Birch gave me a Shiny Chikorita in my AS yesterday after 1534 SRs! ^-^

Here's the live video confirming it:

And here's a full summary picture of my Chikorita: http://gyazo.com/38c7403978c8ccb69bc25fa8fef7269e

If you want a shiny Johto starter, a better idea would look into using the "masuda method" to breed them. It increases your chances of getting a shiny by a significant amount and you can also control more about the babies, such as nature and IVs.

No they are not shiny locked...I got a shiny cyndaquil from Birch on my first try