I'm thinkin this one is beyond the realms of DIY but here goes.
I'm a guitar repairman by trade. Effects come up in conversation now and again (I had some fun building the Omnidrive, the only effect I've so far found the time to build. Thank you Mr. Hollis and Mr. Keen it was fun. Don't worry though, I haven't been selling anybody elses designs as my own. Though I must say quite a lot of my customers would pay good money for custom built effects. I'm starting to ramble here....). Anyway, a number of customers have expressed an interest in tap controlled delay (for delay time that is, tapping foot along to the beat) pedals. As far as I'm aware there are one or two commercially available.
Now my own take on it is that this would probably be fairly difficult (if not downright absurd) to build. I'd imagine it would need to be digital with some awkward detection. My own thoughts were that it might be easier to have an analog delay with delay time controlled with a rocker pedal (I reckon I have the necessary mechanical skills and equipment to build one of those). However, this got shot down by complaints of "it'd be too awkward to tune to the correct delay on stage" and "the control would have to be too course to give sufficient range, making it too inaccurate to be useful". Though one did say that a rocker pedal to control feedback with lateral movement to control sweep would be nice.
In any case, it's a nice thought, and for the entrepreneurial a market. Feasible? Or perhaps my ignorance is a little over-evident. Cheers for taking the time.