In Kiloude City, after defeating the Elite 4, you could challenge your rival whohas in his/her party an Absol holding an Absolite. Is it possible to use Thief/Covet on this Absol before it MegaEvolves and keep the Absolite?
No. Mega Stones cannot be stolen/swapped.
Firstly, Mega Evolving takes priority over using any move at all, so you won't be able to steal the mega stone before it evolves.
>Mega Stones ... cannot be taken via Thief and Covet, cause Trick and Switcheroo to fail if used on or by the holder, and cause Fling and Bestow to fail if used by the holder. Embargo and Magic Room also cannot prevent the use of Mega Stones to Mega Evolve Pokémon. Mega Stones are not consumed upon use.
So it won't work