Normally the preset boards determine what waveforms are selected and send signals to the points in the picture, found on the VMS board. Instead add switches that provide direct control.

These would switch between GND and -15V for on/off. You could also switch between the preset and GND for preset/on toggling.

Either way you can select a single waveform or layer up to 3 waveforms and make any combination. Waveforms can be set per keyboard as well.

The orchestra waveform generators are not using the 'IL' and 'IH' envelope CV inputs. Instead these are tied to GND and -5V for both oscillators. These can be found on the VMS board.

Alternatively the envelope memory IC can have the same thing done to it, to modulate all waveform generators all at once in an easier way. See the Flutes AM mod below.

Instead a custom signal could be fed to these inputs of either one or both of the oscillators. Either a free running LFO or using the (buffered) output of one of the other waveform generators (either of the same oscillator or the second one). Keep in mind you cannot filter this signal as it includes the serial processing of multiple voices.

The envelopes normally go from GND to -5V, with zero amplitude being -2.5V. So it's best to stay between the original GND and -5V to prevent issues.

To keep the waveform DC centered (-2.5V) it's a good idea to apply the normal signal to IH and then an inverted version to IL.

The Flutes waveform generators cannot be amplitude modulated directly via IL & IH.

However the envelope generators use a memory IC that has two similar input, which are also tied to GND and -5V. The -5V can be replaced by an LFO to modulate the envelope output and thus all the waveform generators it outputs to.

BEWARE: the service manual seems to have some mistakes, GND and -5V pins may be reversed.

My only letting the top 2 bits through the sine wave should change into a square wave. This could be done via an 9-bit gate (AND gates or octal buffers) with an enable connection routed to a switch to toggle between the normal sine and the square waves. A0 to A8 would be gated.

Due to the full 16 bits being used, because of the multiple footages, only the lowest footage will be a pure square wave. Higher footages will have an increasing amount of steps.

If all footages need to be pure square waves, then the waveform generator boards themselves need to each have a gate, which requires modifying those boards themselves, though a simple adapter board that sits between the main PCB and the waveform generator PCB would also work.

There are actually 10 sine waves being generated by the Flutes, though only 9 are accessible on the front panels. The 10th "secret" footage is only used for the Chimes preset, thus is fully functional, but could be modded to have a drawbar for your own organ sounds.

Simply take the output from the IF board and route it to a potentiometer of choice. The other footages split off to the custom preset panel, so you could do the same for the new footage. Though you would need to add a new slider or something to the panel...

It is possible to make the two oscillator setup of the E-70 Flutes more interesting, besides the stock detune setting. The oscillators can be shifted from 1 oct down to 3 oct up, however the setting always applies to both oscillators.

The mod points can be found on the FRW board.

This mod does require the cutting of traces, so that only the second oscillator is affected.

You can either add an octave-down switch to F1, that toggles between the original signal and -15V, or you can add a rotary switch with diodes and pull-down resistors that toggles through the binary values as noted in the picture, so you can have the full range of octave shifts.

Keep in mind that F1-F3 are used by the arpeggiator, so having the switch active would mess this up.

The connections between the Key Coder and the rest of the circuitry can easily be hijacked by removing 9 jumpers (how convenient!) and reconnecting everything to a microcontroller of choice. The timing clock signal can be taken from IC7, pins 1&2, and used as an external interrupt. All this can be found on the KCA board.

The E-70 uses -15V logic! So make sure to use proper level conversion, otherwise you'll blow things up. I've used inverting opamps (with high slew rate, such as TL071 and TL074) to convert the +5V from the Arduino to -15V for the E-70.

The microcontroller could forward the data generated by the Key Coder, whilst appending new data from MIDI commands. This way you can keep the keyboards fully functional.

I've added a diagram of the KC data, with timing and counters. This will act as a reference guide to the Arduino Uno code I've written: download. This code is only for the MIDI -> key code conversion as my E-70 project has no keyboards. 
It should work, but there are still some minor bugs that need fixing, these are noted in the code.

K1 and K2 could be OR gated via two diodes to K1 with K2 tied to -15V on the Flute boards (AEG and FWM1/2) to make all notes be played on the upper keyboard outputs, this way all 10 footages can be played for the lower keyboard too.

This mod is required for the Additive Synth mod.

TODO!

With a fast enough opamp this could be done before the demultiplexing/S&H, thus simplifying the circuitry to a single PWM opamp comparator circuit on the sawtooth waveform card output.

The opamp needs to be fast enough to allow the serial processing at 1MHz to pass through unaffected, so the S&H properly samples the PWM waveform voltages.

IC1 envelope generator for the Flutes has the repeat function permanently disabled by having FU3 and FL3 tied to -15V, whereas the Attack envelope generator has these switchable for Attack repeats.

The Flute envelope generator can also have a repeat mode, by replacing the -15V connection with a switch to toggle between -15V and GND for repeat off/on.