They could make Regirock hold the registone, Regice hold the regiscrystal, and Registeel hold the regiplate (cause I don't think the can code Regigigas to know if they have the others in their party). Then you put them in a crafting bench to break them into 2 regidusts (registone), 2 regishards (regicrystal), and 2 regingots (regiplate). then place them in the pattern of the stones on Regigigas's face, the dusts=red shards=blue ingot=grey:
shard shard = Regitrio or Regigem
Then you take it to a forest temple and right click a statue version of Regigigas and now you can fight and catch him.
I know a lot of coding, but I think it would be cool