Is there a way to add short hardcoded subs without reecoding everything?

[M4ST3R]

Hi,

My TV only supports the mp4 container and does not read subtitles.

Then when I want to watch MKV videos with soft subs, I reencode them to mp4 (x264/mp3) with hardcoded subs using ffmpeg.

But I have some videos with some short (forced) subs. It sucks to reecode everything when an one hour video has only a few seconds of subtitles...

Is there a way with ffmpeg, x264 or avisynth to reecode to h264 only the periods containing the subtitles?
It's probably not possible, but I ask...
Ideally something like: When ffmpeg detects no sub, it just mux untouched video. Then when it enters a range with subs, it reecodes. Then switches back to muxing mode,...

I can maybe do that manually by splitting the video in several parts, reecoding the parts with subs and merging them later. But well, I don't want to spend 1 hour to do that...

Thanks in advance.