Trying to steal my thunder, are we... -_-...
j/k. Well, I'm pretty sure that the Shinyness will be determined at the start of combat with Groudon. Afterall, I don't think that big lumberring image you see is its icon, presumabley it gets a smaller one.
I'm gonna go and try a few test captures and note down the stats. If the stats are different then its shinyness would be determined in combat.