Alright, so are we absolutely sure that nothing in the Elite 4 rooms advance the frame other than walking 128 steps? 'Cause I'm getting inconsistent results with roamer respawns.
First I faint all 3 roamers on the map, then save my game right in front of the Indigo Plateau door, facing forward w/ my first party member right behind me, also facing forward. I land on the seed that I'm looking for and advance to frame 227, then I run forward through the first room as fast as possible, mashing A to get past the guard as fast as possible (all to limit the movement of that one NPC in this room). Once in the first Elite 4 chamber, I take the shortest number of steps required to defeat each successive trainer, beat Lance, save the game, and return to the title screen. Upon restarting my game and catching the roamers for verification, Entei seems to have been generated on frame 242, Raikou on frame 247, and Latias on frame 252.
So I restart, faint the 3 roamers again, and return to Indigo Plateau to save my game in the exact same position as before. I land on the same seed, this time advancing to frame 217, and proceed through the Elite 4 building in the same manner as I did previously. After beating Lance, restarting, and catching the roamers once more, this time Entei is generated on frame 226, while Raikou is generated on frame 231.
There is a 15-frame gap in the first run and a 9-frame gap in the second run, and I can't figure out why. Did I overlook something? Do any in-battle actions mess with the RNG? What about potion usage and party order switching outside of battle? Kind of stumped here.
EDIT: Right after posting and re-reading this, I notice that it's a 6-frame discrepancy while I was using a party of 6..... Nevermind.
At least I'm getting some EXP and cash out of this. -_-