Well, I guess I'm back to work on something new with all this with the US release.
Basically, an undub patch isn't going to happen since the music and such takes up over half the size of the game. And image/text replacement the other way around probably would still be a pretty big patch, although I'm not certain or anything yet.
But I have attempted making an undubbed ISO is the most simple way, by just replacing the rev_tengoku.brsar from the Japanese version into the US version of the game, and that didn't work. The game played and everything, but at least in the game I tested it in (Ringside, the wrestler interview) it played the music but not the cues. I don't know if that means it just wasn't loading the cues or it wasn't loading the sounds that were changed since I only tried it with Ringside before I went back to playing through the game normally (Beat up through Remix 5 in about 2 hours total) since I'd like to have as much unlocked as possible before I try anything else.
EDIT: I tried it again, and it does only break the SFX, but only in some games. And if it doesn't work in a game, it won't work for that game in whatever Remix it appears in. And it's not dependent on dubbed/undubbed, so I'm guessing it's related to the index or something in the brsar. Works out like so from what I tested:
General Sound Effects - Works
Packing Pests - Works
Shrimp Shuffle - Works
Board Meeting - Works
Micro-Row - Works
Double Date - Doesn't Work
Donk-Donk - Doesn't Work
Fork Lifter - Doesn't Work
Air Rally - Doesn't Work
Cheer Readers - Works
Figure Fighter - Doesn't Work
Samurai Slice - Doesn't Work
Flock Step - Works
Assembly Line - Works
Screwbot Factory - Works
Bossa Nova - Works
Hole in One- Doesn't Work
See-Saw - Doesn't Work
Monkey Watch - Works
Catch of the Day - Works
Exhibition Match - Doesn't Work (Crowd Does, maybe General Effect?)
Rhythm Rockets - Doesn't Work
Love Rap - Doesn't Work
Tambourine - Works
Tap Troupe - Doesn't Work
Flipper-Flop - Doesn't Work
Karate Man - Works
Rhythm Test - Doesn't Work
Ringside - Doesn't Work
Working Dough - Works
Night Walk - Doesn't Work
General Endless - Doesn't Work
Mr. Upbeat - Doesn't Work
Wake-Up Caller - Doesn't Work
Munchy Monk - Doesn't Work
Lady Cupid - Doesn't Work (Stage Clear does.)
Rhythm Toys General - I think it works, do they have any contant sounds?
Toy Car - Works
Police Call - Doesn't Work
Mr. Hi-Hat - Doesn't Work
Rhythm Fighter - I'll Test Later...
My next plan of action is an attempt at replacing the US text and image szs files in the Japanese version to see if that'll work at all because it sounds simple enough to not work either. After that it's replacing the individual files in those SZS files, which will probably work since image replacement worked before that way. But that's a bit more work and also inolves going through every stage to see what graphics are editied in the US version. In this case the UNDUB would be based on the Japanese version.
EDIT: The plan above didn't work that well either. It might be failing because I'm replacing a lot of files en masse, but when I do so it just stops loading anything from the cellanim folder. Which basically results in the opposite problem from above, you've got the music and signals of the Japanese version, but there's not images in game at all. However, everything in the layout folder seems to work fine so in that case you can navigate the game fine up to when you start playing. It also runs in english when running from the U_riq.szs when replacing text the files in it with the US versions.
For an undub based off the US version that rev_tengoku.brsar replacement would have to work, or I'd need a way to edit the files in the brsar and I have no idea if there's any tools to do that out there, but I haven't found any in all of my searching on my phone today. Since I was really hoping just switching the file would work. Either way I'm going to go back and see if it's just the cues not loading or if it's the sound files that would be different in the brsar that aren't loading.
Other than that there's not much else to say until I'm back home later today.
Here's the dubbed songs wiyth lyrics however.