There are basically two ways to do a delay that does not click when you change the value:
1: Resample the audio so the buffer size changes. This causes a pitch change which is exactly equivalent to what would happen if the distance changed at the same speed (doppler shift).
2: Resample and speed change: This is DSP heavy, but allows the delay to be changed without causing a pitch change while it happens.
Both of these have the delay changing over some time period, and in the case of lengthening the delay the period needs to be long compared to the change in delay time (Otherwise the resample ratio becomes really silly).
The other thing that having this capability would open up, is the potential to be able to do delay panning which would be cool.
Regards, Dan.