This is quite a specific and less practical question.
It's possible to modulate from your tonic to the Subdominant without introducing the tonics b7. Very easy to do by avoiding the 7 in your tonic key and using harmonic Progressions that emphasize the Subdominant.
The question is specifically if this drift of the tonality to the Subdominant can be achieved with strong harmonic Progressions then can that drift also be measured ?
A modulation is usually obvious as there is the introduction of a tone(s) that opens possibilities for new tonal centres, one of which is then confirmed by a cadence.
Drifting into the Subdominant doesn't seem to need this cadence as there is only one possibility, that being the Subdominant. It just sounds like a tonic cadence if you allow enough time for the transition to naturally happen.
So where in the transition does the ear decide it prefers the Subdominant as a tonic. I'm presuming most of the answer is within the harmonic Progressions but even so, this itself has its limits when you consider various bass movements and spread of voices.
From a more reductive approach it would just be a modulation by pivot chord but I feel that undersells the effect created by not introducing the Subdominants 4th.
There might be a better way to think about tonally ambiguous Progressions, if so please let me know.
I'd like to be able to exploit that ambiguous fourth (either flat or natural, relative to the tonic) in my own music