ffmpeg – Replace MP4 I-Frames with a placeholder

I have an MP4 file which I want to manipulate on a hex basis, without re-encoding. More specifically I want to replace the I-Frames of the MP4 file with a kind of placeholder code.

What I want to do with the MP4 file:
The video.mp4 is basically a hex file which I want to manipulate as the following.

-video.mp4 viewed as hex-
I-Frame-Code
….
P-Frame-Code
….
I-Frame-Code
….

What I want to accomplish is:
-video.mp4 viewed as hex-
Placeholder-Code
….
P-Frame-Code
….
Placeholder-Code
….