Taking advantage of your post, I come to instruct you things I discovered:
MassIsoUpdate: I don't know if using this program to import files larger than the original in an iso would cause bugs when playing on the real console.
Makou Reactor: When using the program to use in relation to the PSX / PS1, be careful when doing a lot of editing and saving the iso. Never do that. Always export the Field.dat file and test if it is bigger than the original, then yes, you use the MassIsoUpdate program at your own risk of causing a bug in the iso playing on the real console.
If you have any success playing the iso, having used MassIsoUpdate to import Fields.dat files larger than the original and not causing any bugs, none at all, let me know in the comments or via PM, why it helps me and helps several people.
I'm asking this, because at the moment I don't have a PS1 at home to test my iso with files larger than the original.