@Blindsmyth Thank you, so with a cut off MIDI/power section it appears a direct 5V wired in supply can work. However it's a possibly different situation when the board is intact. It's unclear to me what might happen with the board intact (not cut) because you have now power regulation circuitry which will be floating on the input side and powered on the output side.
Looking at the power regulation / supply schematics you can see a few obvious places to think about. Disclaimer: I'm no expert and my opinion is probably not so trustworthy! But here is what I see:
It appears the barrel jack feeds into an AOZ1280 buck (switching) regulator programmed by the external resistors to provide 4.96 V. This feeds the +5V supply rail on the Axoloti board.
The only other risk point I see is the STMPS2161 power switch chip which provides the power feed from micro USB socket. Looks like a nice little chip!
Of course maybe I missed something!
In both cases, direct-wiring 5V power would supply potential to the output side of the chips while the input side is floating or maybe low. Looking at the data sheet, I think the STMPS2161 would be safe: it says,
When the switch is OFF (disabled through the EN pin), or when the STMPS device is unpowered (VIN = 0 V) the switch behaves as an Hi-Z at the output pin, ensuring that no reverse current will flow into the device when VIN< VOUT
So unless this chip receives an erroneous / incorrect EN signal there is no problem.
What about AOZ1280? Another really nice chip. Soft-start/inrush limiting; overcurrent protection, etc. This too -- if I am reading correctly -- has an enable (EN) pin which in the Axoloti schematic is linked to the barrel jack input via a 47K resistor, so that power input automatically enables the chip and begins soft-start process. The IC should be in disabled mode then when no input is applied there. But the datasheet does not specify what exactly the "disabled" state means except that the chip is not operating. I do not think you can trust that the output pin would enter a high Z state; in other words reverse biasing the chip by feeding 5V power directly to the 5V rail on the board has unclear consequences, from what I can see.
So in other words it might not be a wise idea to direct-power with 5V unless as Blindsmyth has done, you have broken the Midi/Power part of the board off. Have I answered my own question?
But the AOZ1280 is such a beautiful chip, I can't let it go to waste. I think I will just work a 9V supply into my system and use it!!