I'm wondering if anyone has tried to modify existing FEH software. I've read several posts of folks wanting newer model year software installed in their vehicle but can't find any owners actually attempting to "tune" their own.

Tuning software is incredibly common in many enthusiast circles, albeit for performance. There are several programs available that enable owners/tuners to connect to vehicle's computer via laptop and modify virtually everything. Websites devoted to this effort are everywhere.

I remember seeing video of a Japanese gentleman that hacked his Prius to allow cold EV starts, longer EV, etc. He improved his mpg but obviously voided the warranty.

Don't get the wrong impression here. I have no clue how to do any of it. But I'm about to purchase a GMC Syclone and plan on eventually tuning my truck. Obviously FEHs are light years ahead of 1991 automotive technology, but maybe someone here has messed with FEHs a little?

 

How to "tune" the software and/or rewrite the control software is beyond my knowledge... however, yes, of course, it can be done. The cases I've seen, the original software was not changed, but was bypassed, and something else was put into its place for testing and experimentation, so the original operating system could be reinstated at any time.

Nothing commercial can be purchased that I am aware of, but you can hack away at will.

Look up MIMA which is manual control for the Honda Insight.
I have driven one FEH ( not mine ) that was hacked with a hand-held control, ( like MIMA ) and I could add assist, or charging at will, stop it at will, and I could bypass the battery SOC limits by "fooling" the car to think one thing, while doing another. It did not change anything on the gas engine.

It's possible, but not easy, and you really need to know what you are doing, and need to know the CAN languange and how to intercept, and transmit CAN signals.