I know that the weather trio and supposedly Deoxys are shiny locked, but I'm currently SRing for shiny Mesprit, before the Elite Four. However I'm nearing 3000 SRs now (a total of 4 days work considering I have college in between) and I was wondering if I'm wasting my time. I don't just want to give up after spending so long on it though, so if someone could find out if you can get shiny legendaries before facing the Elite Four for the first time that'd be great. c:
I don't know why you think that beating the Elite Four could change the shiny form of Mesprit. The only shiny locked are Groudon, Kyogre, Rayquaza and Deoxys, as you said.
Anyway it's sure that Mesprit can be found shiny, I think it doesn't matter when you catch it.