The problem with digital is the software/firmware to run it. Anymore its easier and probably cheaper to buy some kind of digital effects processor.
A short delay with delay time sweep (what flanging is) isn't all that hard to implement in DSP, if you have experience with assembly or C language.
Several DSP manufacturers (Analog Devices, TI etc.) have inexpensive DSP evaluation boards with assembly language editors/loaders which you can play around with if you're really interested in "rolling your own" DSP. They are kind of fun.
Other than that, you are looking at R&D expenses far beyond what a new effects processor would cost.
I have no idea if there would be enough of a market to pay for the expense of developing a board level DSP that could be added to an amp or housed in an external enclosure. Might have to think about that one...