Audi COPs - where do you get a tach signal from?
 

The Audi/VW COPs don't have a tach signal pin like the Toyota COPs do. See the pin diagrams for both types on here:
Mazda MX-5 COP/Coil-On-Plug Install – Motorsport Electronics (motorsport-electronics.co.uk)

When using Audi/VW COPs with a Megasquirt on a Mk1 MX5 / Miata / Roadster, where do you get the tach signal from? Do you get it from somewhere else and wire up to the back of the gauges?

You need to wire an output from your ecu to the tach’s signal wire, and set your tach out to this output in the software. This is typically done when eliminating the mk1 ignitor, tying the blue/yellow (?) and black/white (?) wires together, thus connecting the ecu to the tach.