Originally Posted by Fat Misdreavus
You can trade a Pokémon holding an item to R/B/Y. When you trade it back to G/S/C, it will still be holding that item. In the case of Scyther holding Metal Coat, it simply won't evolve (either going from G/S/C to R/B/Y or the way back).
No. The game doesn't allow you to enter the Time Capsule Pokémon if there is a Pokémon holding an item.