If I were to implement tap tempo on the beatstep, I would consider using use the start and stop buttons (maybe while holding shift?). If in play mode, I would use the play button, otherwise the stop button.
In order to avoid the shift button, I would add a threshold so you enter the tap tempo mode by hitting the button a few consecutive times in a reasonable time (the first 3 or 4 hits are discarded - they are required to enter the tap tempo mode - then the following hits are used to calculate the tempo). To exit tap tempo mode, you would have to wait a reasonable amount of time without any hits (let's say something like 5 seconds?) or press the other transport button (so, if you are in play mode and you're tapping the tempo by pressing play, if you hit stop you would both stop the sequence and exit the tap tempo mode).
I think this way it's easier to tap the tempo, since you don't have to hold down any additional button, you can keep the other hand free to do something else. The drawback is that you need a few more hits to set the tempo. Probably this approach is best while playing.