This is a little bit off topic here, but I think I figured out why some Pokemon is GSC have Charm as an egg move.
If you look in the Spaceworld demo's disassembly and go to Level Up movesets it reveals that both Cubone and Marowak could learn Charm at the time! From a technical standpoint then, if breeding was implemented in the demo, you could breed with them to get Charm on Bulbasaur and Snorlax, then for Oddish you just chain breed with the Charm Bulbasaur.
What's not answered though is why some are version exclusive.
By the way, I also heard that in the MYSTRY Mew save file there was a Blastoise registered in the Hall of Fame. You think it would be possible to recreate it? Sounds like an interesting project.