I'm going to take a shot at this because the way I'm reading it, you could set up the expression pedal to work on the mix control and the tap tempo is controlling the speed of the delay. This would let you increase or reduce the amount of delay with the expression. Off in heel down and how much max at toe down is up to you. You'd then also have anything in between to work with.

I actually think the expression pedal control on the M13 is much better on the DL4.  I used a DL4 for many years on my pedalboard, and one thing that I always hated was that if I tapped in a tempo with the pedal in the heel position, it wouldn't be there in the toe position.  The way it is on the M13 is much more usable for me with the way I use the delays, because I rarely will want to change the delay time with the expression pedal, but I will often want to change other parameters without affecting the time.

That being said, I do see why you would want to do what you're saying.  But I don't think that having a different global tempo for the heel and toe positions would be a good thing unless there was a way to choose not to do it also.