I would have thought that would have worked unless it was running too high of an initial voltage. 500 mA should be more than enough to work. It might require a "regulated" power supply that will keep the voltage between 5V and 12V regardless of a load being present.
That said, using the WiiM Pro as a trigger is a logical next step.
I'm running a variation of this idea to voice control both my bedroom and office systems by triggering the power conditioner(s) on and off.
Let us know how the WiiM works out.
- Jeff