I've been playing through the last week or so and I'm finally at a part where I can go after the legendary "dogs". I've found the other two but how did you set up the dynamic for the roaming one?
An easy way to find it is stay on a patch of grass close to a building (i did this on cerulean, right before the bike road) so you just keep entering and exiting that building while checking if the legendary dog is in the grass.
Also, if you already have one of the dogs, you can use a repel to only find the legendary. If you're using an emulator, savestates come in handy as you can just save before checking if the dog is there, if it is, you load and change your first pokemon to one that can keep it from running away. I don't know if this works with in-game saving too, though.
That is a really cool idea, and totally worth putting in a hack... the rebirth ball... I like it, would it drop them to lv 5 or drop them a certain number of levels? you could make several legendary (i wouldnt say all) require the ball... or even have a lv 100 fight that requires the ball... so you have a hard time beating it then catch it with the ball and (if if drops to lv 5) you would have to start all over lol
if we wanted to do some math, it could drop them a random large percentage of their current level... like 60-95%... so a lv 20 could end up at 2 or 8... a lv 100 could end up at 5 or 25 or even 40 (so their current lv would not depict what level they end up at after you catch them)
it would be a bad choice of ball for low level pokes but a great choice if you wanted to train legendaries... and I have no qualms making it a x2 (ultraball) because of its "negative" side effect (especially on lower level pokemon)
I have several members of another forum working up a story for a hack they want me to make... this would be a great addition to it (as it is its own story, it can have new elements)
As far as story goes, I think it could use some scientific / ecological project; something like "Some Pokemon in the wild are getting too strong and becoming a menace to humans and pokemon alike in some regions. We've developed a ball that can weaken them enough for a young trainer to tame them so they're no longer a threat". Given a story like that you can easily understand why pokemon captured with that ball get so weak, and so you won't need too much "in-game" explanation as to why not to use them on weak pokemon.
The math sounds good enough, and it would imply that the highest level pokemon could be found a bit earlier in the game, as they would get a reasonable weakening with the ball. i'm wondering though if it would be good to de-evolve some pokemon too (only the ones that evolve by leveling-up) as to make the rebirth effect more realistic.
Edit: of course, all of this doesn't need to necessarily be the main focus of the story, but it could be added as a secondary-still-kinda-necessary objective, as I said, there could be certain pokemon (not too many because it would get old quickly) blocking the road at some points, they can even be the starters you didn't get to choose at the beginning of the game.
Edit2: this can also be used to need some backtracking, if you can get free Rebirth balls at a lab whenever you've run out of them, since the "negative" effect will keep most trainers from over-use them anyways, specially if they actually want their lvl 40 fearow to stay that way. And the number you get could get higher after each RamPoke (Rampaging Pokemon) you captured. Say they give you one ball to start, and right after you find a Venasaur blocking your way, so you have to use it, then you go back and they entrust you with 5 new balls because you captured a RamPoke, and you can use the ball in any pokemon you want, but if you get a new RamPoke and go back, they'll give you 10 balls this time, and so on. Or instead of backtracking you could add "aides" at some cities or maybe use Oak's computer (or add a new one if it's possible) to report your progress and get the balls. If you make it so there are only 2 or 3 RamPokes in the way, you can just make them start selling you the Rebirth-balls after you captured the last RamPoke.
EDIT:
maybe there is a Safari zone "like" event where you go in agians a bunch of REALLY high lv pokemon, (like 90-100) and you can only use these balls, maybe they are rare and the only way to catch them is in this event... (like chansey in the current safari event)
This could be a really cool add-on, but I still like the idea of making the Rebirth-ball have a certain importance in the story, at least at certain points where it's more of a must than a choice to catch a pokemon with that ball. This safari-zone could be like a "quarantine" where way too many pokemon have gone on a rampage so they're isolated and only certified trainers (with a few badges, pokemon league, or maybe just a special permission from the professor) are allowed to go in.