On those bikes does the mileage showing on the gauge get a feed from ECU or is it stored in the gauge itself ?

Cool.
Buddha.

Sorry, can't confirm for bikes, but on cars like e.g. my wife's Hyundai it's stored in EPROM on the gauge PCB. Don't ask how I know.

I'd be surprised if bikes work differently, but you never know.

Yea its in the gauge on bikes too, except for very few exceptions (Think the ZX10 and 14 based bikes are in the ECU). However It is resettable - which I presume is to ground 2 wires - for example the trip meters are reset by grounding them, the main mileage is also in the same way, but we have to find the 2 wires in the plug and ground them - most likely grounding them all to each other with the gauge off the thing likely wont hurt a thing.
Cool.
Buddha.

SV portal guy nails it -
Remove the 93C46 chip and flash the mileage that suits you. Correct it before using Tachosoft 23.1. It's easy.
Looking up what that tachosoft needs and will do it.
Cool.
Buddha.