It would consist on pressing the block button 1 frame away of an impact...
That would be the tightest just guard window I know of.
But tbh I like the way it was done in Soul Calibur more. There, you had to press block for no more than 4 frames, and then, upon block release, your character will enter "just guard state" for a couple frames more. You must not be in a state of blockstun for it to work.
This ensures that you can't just guard without intention of doing so, and that you cannot just sit there and block until you confirm last move in a blockstring for JG punish.
That said, I agree with the following:
I say no. Either this becomes something that people just get lucky with and it adds to the chaos of the match instead of adding to the strategy, or it becomes consistently used and it adds a pointless execution barrier to the game.
While the former issue can be mostly solved by implementation described above, the latter is my main concern. Although since such implementation requires hard read for most things that become punishable (fast, not advantageous strings) and just make other moves potentially less oppressive (slow and reactable, but plus)... I think you can build the game around it, after all. You just need to switch off your inner NRS while designing such a game.
Honestly now, if you ask me, I think that MK should step back from frame game for a bit and focus on hitbox/hurtbox (aka spacing) games more.
And speaking of raw execution (which is somewhat off-topic, but w/e), NRS already make it harder every release, but harder in a pretty meaningless way, as in: mostly not because your character has so many more options and can do more in the same timeframe ("mostly" being a keyword though), but because MK9's 344 becomes IGAU's b2u2d3 becomes MKX' f3~bf[4],f,f+blk,[!4]. Like, what the fuck is the purpose of this?
This is not what makes a competitive game, it's all about something that lies behind it. Something that doesn't require esoteric execution of moves, and there are prominent examples of that these days.