I guess what you'd have to do is program a preset where the exp pedal is assigned both to feedback control and delay time. Once you're through with it (piece of cake), you have to set a minimum value for dly time at heel down position, and a max value for toe down. That way you'll be navigating from one delay time to another. Be aware that you'll be hearing the speed-up/slowing down effect while moving from one position to another, unless you use the dedicated delay type developed by eventide which is DIGITAL DELAY (it won't work with other delay types such as Mod Delay or Vintage Delay). Try starting off with preset #5:1, you'll see what I mean.
