Custom Import button

I have a custom toolchain of Unity extensions, and I'd like to work I2L into it.
This means I need a simple function to call the Import_Google(eSpreadsheetUpdateMode.Replace) which exists in the I2L Asset inspector. It seems that a lot of the logic is dependent on the way Unity uses custom inspectors, and was hoping someone could shed some light on the steps needed.

I made the Import_Google function public, to just test it, but it seems like some needed callbacks are not happening if it's not properly being shown.