I just was changing the upper death boundary, but what Dantarion is saying about just changing animations is interesting itself. Unfortunately, I have very little experience with this sort of thing; just looking at Mushroomy Kingdom 1-1 in BrawlBox isn't making too much clear to me. I am seeing some stuff...
I'm pretty sure the animation for the stage moving is StgMarioPast00_Ashiba_locator which is 7200 frames long. In this animation, two bones (the promising X_move and the more mysterious haikei_locator) increment their translation X values each frame for most of the animation, growing more negative as time goes on (and they always have the same value on any given frame). Frames 1, 3449, 4649, 5845, and 7041 all have their translation X values highlighted in yellow in the "advanced model editor"; I think that makes them keyframes. I actually think I might have this figured out; I'm going to try cutting the translation X values of all of these in half. Here are the appropriate values per keyframe:
1: 0
3449: -846.6755
4649: -1141.342
5845: -1435.027
7041: -1728.711
I actually just tried that in-game. The result was that it mostly scrolled at half speed (though things like the background moved at normal speed, making it actually a better version than the slowdown method), but at a few places the stage jumps in very undesirable ways. A better approach might be doubling the length of the animation and doubling the spacing of the keyframes. I don't know if the game will barf on a 14400 frame animation, but we'll have to find out, right? This puts the keyframes at 1 (by executive decision not to double), 6898, 9298, 11690, and 14082.
The result was that one undesirable jump still happened, and then where the second one used to happen the looping seemed all but broken. It took an exceedingly long time (several minutes) to loop, which it did via warping.
So, I feel like I'm really close here, but I don't quite have it. I think this sort of thing could be pretty useful if only we could work out these... quirks. Hopefully this chronicle of what I was doing will prove useful to someone inspired to look at stuff further.