I'd rather avoid dealing with legendaries except in circumstances where we may find ourselves in a position to help them from Team Rocket's predations.  In any event, I doubt it would be so simple as "catching" them.  Considering how many poke balls it can take just to catch one in-game, and the rarity of them in this roleplay, it'd be next to impossible.  At best I could see convincing one to join you of its own free will.  If Team Rocket is upsetting the natural balance of things, I imagine they would get pretty riled up.