To make this useful for potential Rachio 3 users in Australia, I’ll update this with the pictures showing progress of disabling the 900Mhx Radio
Guided by the FCC diagram
I opened the two star-drive screws on the rear of the case, allowing the unit to gently prise apart.
The updated board has the same apparent layout.
Noting the upper left corner (transilluminated to show what I presume is the antenna:
Some mini-cutting pliers employed to sever the antenna circuit.
Now to obtain a 24v AC Australian power supply and discover whether I have bricked the unit.
Will keep posted, if only for your bemusement.
Advice from people with actual electronics knowledge very welcome (eg. is this adequate to disable to 900Mhz signal?)