Well, I don't disagree to what is being said, because it all makes sense.
Back in my band days it was the conductor who controlled the tempo and also his body language controlled the feel and also the expression and dynamic loudness.
(Of course it was all written in the music, or we were allowed to edit our music to taste adding notes, whatever...)
And it was through practice that things like the speed of the music were also keyed to what the conductor wanted from the band. And it was important that we learned more to follow him should he choose to express a song differently.
So, the tempo was not geared toward a number of measures, or even toward a whole measure, but was definitely on a note-per-note basis. And this is what we did as musicians (to learn to follow).
Now obviously the midi keyboards can follow explicitly, even to a more finer point than humanly possible. However, in midi a tempo map is used. Now I don't know how to incorporate a midi map into your particular setup. But I do know how to create one.
In most DAWs you can set the tempo to a certain location, and it doesn't even have to be at the start of a bar in some DAWs. When you set the tempo marker, there is an option to gradually change tempo up until the next tempo marker, etc.
There is also a feature available (at least in Reaper) to modify the tempo on a curve to better fit how you want the tempo to vary. Once you create the tempo map you can import it into any sequencer.
If you can get this tempo map to vary the tempo of the keyboard, then you've accomplished what you want and it will have the fine changes in tempo included within it.
If you can't get this tempo map to vary the tempo of the keyboard, then I guess using registrations will have to do. However, I have noticed (I believe) that there are some style endings that do slow down toward the end of a song. So maybe it's possible to use one of those or make up your own.
I know the theory and the mechanics, but I don't know the keyboard intricacies.