Whoops, forgot I asked here.
I extracted the ISO to a directory, created the file, then created a new ISO from that. I figured that whatever sort of copy protection would stop that from working on an actual Playstation wouldn't trigger with an emulator.
That may or may not work depending on how you built the disc image. If the whole thing was built in strictly Mode 1 or something, a good bit of the data will probably be trash from the hardware's or emulator's perspective. If you're looking at this as a learning experience, I suggest using the opportunity to learn how to use PSX CD-Tool, or the LUA Interface kit that grew out of it. Using those, it's fairly easy to extract the contents of the entire disc, and rebuild it while maintaining correct formats, and inserting files however you wish and in whatever order you like.
Would this LBA/Size list be in TITLE.EXE, or am I misunderstanding what "main executable" refers to?
Typically the main executable is named with the Sony-issued serial number, e.g., SLUS_000.00 in the root of the disc. However, open up SYSTEM.CNF in a hex or text editor. That will tell you what executable loads when the PSX starts up the game. If there are multiple executables, that may not be the "main" one, though. For instance, TITLE.EXE may load immediately to display company logos and such, and then it hands off to the main executable when you press start or choose an option from the title menu.
That said, any PSX executable file will most likely have the file listing inside it, assuming one exists, and it may be in other files. In order for the game to function, you will probably have to apply any required corrections to all copies anyway. So find it once, and then search for it elsewhere. One game I've worked with has hundreds of files that contain executable code (but only one proper EXE), and something like 60-70 of them contain an LBA+size listing for the entire disc. Every time I rebuild an image of it with new changes, the listing must be updated in all of them.