It would be fantastic if someone could find the solution, I'm also very interested, I've been trying to find a solution to translate the game into my native language, but I've only found solutions for MGS II, MGS III... Nothing for MGS 1. I will follow this post carefully.
Anyway, here`s what you need to do on the PS1 version: 1. Open the file on hex editor(obviously). 2. Add the first 16 bytes at the start of the encoded (VAG) file. These bytes must contain zeros. 3. Right on byte 0x1, type "02". 4. Save the file.