How do you plan on doing this?
If they can only be caught, not bred, then there is no way to obtain a move that would be learned past level 50.
The only way this would be possible is if the move was a TM as well, in which case it would be allowed.
There are certain moves a Pokemon will learn after level 50 that a previous form can learn at an earlier level. It is not only acceptable, but highly recommended that you take advantage of this fact.
EDIT: So as to not cause a bump, here are the final rules I'm seeking approval for:
Rules:
0. 16 participants is the current max. If more interest is expressed, this will be increased to 32.
1. Battles will be conducted in a Singles Battle format, with Pokémon up to level 50 permitted.
2. All Pokémon must be captured from the Safari Zone or Great Marsh of any third- or fourth-generation Pokémon game (Ruby, Sapphire, Emerald, FireRed, LeafGreen, Diamond, or Pearl).
3. As a direct result of Rule 2, no Pokémon may have Egg Moves, or be in any ball other than a Safari Ball. Moves learned from Move Tutors are permitted, such as any third-generation game learning moves from the Battle Frontier Move Tutors. Moves learned by TM or HM are also permitted, as are level-up moves learned at level 50 or before.
4. Pokémon caught in a third-generation game and evolved in that game are not permitted to have fourth-generation abilities. Likewise, Pokémon that do not evolve and come from a third-generation game must not have a fourth-generation ability. (Pokémon caught in a third-generation game and evolved in a fourth-generation game are permitted to have fourth-generation abilities, should the ability switch upon evolving.) The following are thus not permitted in this tournament: Miltank with Scrappy, Pinsir with Mold Breaker, Scyther with Technician, Shuckle with Gluttony, Stantler with Frisk, Tauros with Anger Point.
5. Although available in the Safari Zone of Ruby, Sapphire, and Emerald, Wobbuffet is not permitted to be a member of any team taking place in the tournament. Although available in the Safari Zone of FireRed and LeafGreen, Chansey and Blissey are not permitted to be a member of any team taking place in the tournament. Dragonite, a Pokémon that evolves from Dragonair at level 55, is also not permitted, as a result of rule 1; despite an event that has a level 50 Dragonite, that is not able to be caught.
6. No two Pokémon may hold the same item. (Item Clause.)
7. No two Pokémon may be of the same species. (Species Clause.) Previous and alternate evolutions are permitted.
8. You may not choose a move that would intentionally put more than one of your opponent's Pokémon to Sleep.
9. The tournament style will be “Single Elimination Format”.
10. All matches are to be played on Wi-Fi, as a direct result of rule 2.
11. Only Pokémon legally caught in Safari Balls, in the Safari Zone, are eligible, as a direct result of rule 2.
12. Use of the Pomeg glitch to circumnavigate any issue is forbidden. Likewise, using any game-altering device to catch or otherwise change the stats of a Pokemon is forbidden.
13. As always, pairing will be random.
14. I will take a slot in the tournament only in the event that one slot remains to be filled, and no further people seem to express interest.
Below is a list of the final legal evolutions of Pokémon that can be caught in Safari Balls, and thus used in this tournament:
Hoenn Safari Zone (R/S/E):
Ariados, Ampharos, Ambipom, Azumarill, Bellossom, Dodrio, Donphan, Forretress, Girafarig, Gliscor, Golduck, Golem, Granbull, Gyarados, Heracross, Houndoom, Ledian, Miltank, Noctowl, Octillery, Pinsir, Quagsire, Raichu, Rhyperior, Seaking, Shuckle, Stantler, Sunflora, Ursaring, Vileplume, Xatu
Kanto Safari Zone (FR/LG):
Blissey, Dragonair, Dodrio, Exeggutor, Golduck, Kangaskhan, Nidoqueen, Nidoking, Rhyperior, Parasect, Pinsir, Scizor, Seaking, Slowbro, Slowking, Tauros, Venomoth
Sinnoh Great Marsh (D/P):
Arbok, Azumarill, Bibarel, Breloom, Carnivine, Drapion, Exeggutor, Golduck, Gyarados , Kangaskhan, Noctowl, Parasect, Roserade, Quagsire, Sharpedo, Staraptor, Swalot, Toxicroak, Whiscash, Yanmega