There are no secret files. NTFIC1 is correct. There are also numerous FMVs that use the lego model design, so it was a finalized method. The truth is they designed the game to run as barebones as possible (probably a decision made at the start, because they believed the system would be N64 - and they had just come from the SNES, where memory and filesize considerations are massive). They likely realized that they could have higher resolution models (as FF8 had), but by that time it was far too late to start changing everything.
You have to realize that the team was totally unprepared for the power that the Playstation has over the SNES, and the capacity that CD-Rom has over a cartridge. By the time FF8 came along, they'd learned many lessons and knew what was and was not possible. They'd also learned what worked well and what didn't - hence the FF8 audio functions are far better than 7's - and FF9 dialogue functions are far better than either.