It would be hard
You can see their pictures during the game, you don't need to see the actual pokemon.
Manaphy's picture is at Mr. Backlot's house.
Dialga/Palkia are in a book at Celestic Town, I think. Talk to that old lady there.
Manaphy, like pokemaster said, is found in a book at the rich guy's mansion.
in pokemon platinum/pearl, palkia must be fought after fighting cyrus for the last time.