Can it be done?

Assuming the gauges should work fine?? XH and XG both have electronic speedo, fuel sender should be the same, and water and battery volts should also be the same yes?
My XG has Smartlock. Again, assuming this shouldn't be an issue.

Essentially, this would be the same as wiring in a EF-EL cluster into a EB-ED.

Can it and has it been done?

Yes.

Yeah.. been doing a bit of reading on this and it seems possible. Have to use a XH ECU for tacho and temp signals(?). Not 100% sure if that is the case for a XH cluster because I believe the XH engine is exactly 100% same as XG?? Yes?
And XH (non XR) also uses the log manifold same as XG, so won't run rich.

What is the down side to having a XH ECU in a XG? Any issues to be aware of?

Just need to clarify some of the things I'm not sure about and I might just have a shot at it.