The older flangers where built different than the one's made today. For complete details of the chips and circuit design one should examine the schematics of flangers built years ago before 1980. Originally zero-flanging was done using a pair of analog tape decks. You may want to pickup a reel to reel tape deck and experiment with it. There are millions of things you can do with them. Another nice unit to use is an old tube Echoplex. Analog has its own particular sound.
The thing with these units is a combination of mechanics and the stretching of the tape can create dropouts, wow and flutter. Then add in some vibration of the tape itself as it slips and sticks against the metal surfaces of the heads. The output will create sideband frequencies separate from the frequencies of the cyclical variations produced by mechanical imprecision, together with the tape vibrating the sounds will go up and down in frequency. The tape produces harmonic distortion and the result is the addition of some even-order, more odd-order and a spattering of inter-modulation.
You might think about studying electronics and mechanical theory's from the Eisenhower era.
You have to reintroduce the original dry signal back into the signal chain. You have to take a look a the equipment you have and see how you can split the signal and bring it back. You'll need two signals. One to run wet and one to bring back.
You should consider the possibilities of linking various tone sources in series and parallel configurations. This will open up vast fields of sonic territory. The interesting alternative is to split the guitar signal, with one side going directly to the distortion, and the other directly to the effects, then mix the outputs of the effects and distortion together before the amplifier.
Example: I use 8 El84's for my pre-amp. Those outs are routed to numerous other pieces of rack mounted equipment, which allows me to split the signal and bring it back anywhere, then adding a Midi switcher allows a change in the signal path in real time.