On most Chase Bliss pedals, the MIDI jack can be used with a standard momentary footswitch to toggle on/off the left-hand footswitch. This is vital for tabletop use of pedals like Blooper, where the left footswitch starts/stops recording and overdubbing.
On Ribbons you frequently need to tap the Touch footswitch for modes like Disintegration Loop. The hack I’ve implemented is using the EXP jack mapped to the Touch switch, with a clicky toggle footswitch. One click to start, one to stop. Unfortunately this uses up the EXP jack, and the clicky footswitch is kind of gross to use.
This solution could “double” the left footswitch, rather than taking it over as it does using the EXP jack. It would work great for all Touch modes.
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The MIDI input jack on our devices follows the MIDI Association’s TRS MIDI spec and uses an optocoupler for proper isolation and to help prevent ground loop issues. Because of that hardware design, it can’t be changed in firmware to also act as an external footswitch input.
Some older devices from other companies used their TRS jacks in less standardized ways, often to support things like external tap tempo or switch control. But as TRS MIDI has become more standardized and more commonly used for external control and clock sync, combo implementations like that have become less common.
There is a discussion on this thread about various ways to get certain sustain pedals to work with the cv/exp input or create your own custom footswitch box.
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Understood. Yes I am on that thread 
I posted about my clicky switch. It does work.
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