I have no idea if the seed affects the catch rate of the pokeball, but don't you have a catcher with spore? That'll increase the amount of times you can attempt a catch, plus if you clone the catcher and have two of him in your party, that doubles your chances, because if the first gets hurt, you switch in with the second. I have used two false swipe/spore Brelooms on some legendaries.
The catch rate of the pokeball is a fixed number, you can know the average number of tries it will take using
THIS page and a scientific calculator. The pokeballs multipliers can be found
HERE, the catch rate of legends is 3, but you can see for yourself at serebii's, psypokes' or bulbapedia's pokedex. However, I'm pretty sure that the seed affects in one way or another the number of tries it will really take for the capture to be successful.
As for a good catcher, may I suggest you take a flawless Jolly/Adamant Technician Smeargle to...
... the Old Chateau and scketch a Gastly's Mean Look (DP)
... the Victory Road and scketch a Lapra's (Lv. 43) Safe Guard (DP)
... the Cerulean Cave, use Safe Guard and scketch Parasect's Spore (HGSS)
... the Bug Catching Contest, use Safe Guard and scketch Paras' Spore (HGSS)
... the Bug Catching Contest (Thur./Sat.) and scketch Nincada's False Swipe (HGSS)
... Move Deleter, delet Safeguard - Move Tutor, remember Scketch
... And
... the Bug Catching Contest (Thur) and scketch Dustox's Moonlight (HGSS)
... Or
... the Sendoff Spring and scketch a Bibarel's (Lv. 43 or above) Super Fang (DP)
... Or
... Route 10 (Hoenn Sound) and scketch Linoone's Odor Sleuth (HGSS)
Super Fang takes 50% of the current HP, no matter what (though it kills if the target has 1 HP)
Odor Sleuth makes Smeargle hit ghosts
Moonlight recovers HP
Ugh... I kind of went into a Smeargle spree yesterday... *hides*