As a test, I deleted the first 8 bytes of FIELD.BIN, ungzipped it, noted the references to the three MD1STIN.* files (.DAT, .MIM, and .BSX), noted the references to the three MOGU_1.* files, and overwrote those of MD1STIN with those of MOGU_1. In other words, I told FIELD.BIN that when it wants to call MD1STIN, it should point to MOGU_1; this should make the game start at Mog House, rather than the Sector 1 train station.
I then gzipped it again, added the original 8 bytes to the beginning of the file again, renamed it to FIELD.BIN, launched CDMage, loaded a pristine copy of FF7 Disc 1 (as a Mode 2/2352-byte image), and imported FIELD.BIN directly over the top of the existing one. Probably due to different gzip implementations, my file was 2 KB smaller than the original, so CDMage asked if I'd like to pad the file with zeros, to which I responded no. It successfully imported the file (to the same LBA/sector as the original), and I saved the new image as a new file (in the same format I loaded it as, Mode 2/2352-byte image; I also told it to ignore sectors that couldn't be 'converted', as otherwise, it would immediately error out...I assume this is related to FMVs and Mode2/XA data or something). I then launched PCSX-Reloaded (a PSX emulator), loaded the ISO, and it worked fine. However, once I hit NEW GAME, the emulator crashed.
My first thought was that the new FIELD.BIN's different gzip compression could be making FF7 angry...quite possible, of course. So I tried simply loading a pristine copy of FF7 Disc 1 in CDMage, in the same way I did earlier, and then saving another copy, without modifying it. This yielded the same result. For some reason, something CDMage does when saving the image corrupts something, and this something isn't a problem until FIELD.BIN itself is loaded...even if FIELD.BIN isn't intentionally modified.
Does anyone have another program that will allow me to insert a file at a specific LBA, and won't cause the issue I'm experiencing?