RECORTEC made these 'add-on' counters many years ago for professional decks.
They used one of two methods -
1. An extra low-friction roller with around 2/3 wrap to ensure good contact in the 'tensioned' zone of the transport.
2. Or an add-on 'W' roller arrangement which provided local tension to ensure consistent friction over the count roller.
You could go deep, but that would be machine dependent - counting the tach from the tape path - hiowever you must remember the diameter of the supply & take-up reels will change as the tape moves - so the tach will also change if derived from those.
The pinch roller & capstan are only reliable in forward or reverse play modes - so that has limitations as well...
Also consider possibel slip when accelerating or braking - differs with large or small diameter tape rollers.
You could try really high-tech - and use something like an optical mouse - focussed on the tape... (sounds like fun) --- @dougp typed it as I was preparing the post !!