I'm not concerned about power usage, but about the necessity to supply power to the device, which requires dedicated hardware. Power supplies nowadays are indeed quite small, but one of them would still probably be bigger than the device itself.
The power supply required is intimately related to the device's power consumption. The kind of device I'm imagining could run from a simple walwart, power-over-ethernet, or directly from a low-voltage outlet if LV wiring was available (as is becoming more common--take for example AC outlets that now include a 5V USB charging port).
This hypotetical device and a POS have completely different (and actually opposite) design goals: a POS must act as a wallet but doesn't require full node capabilities, while this project wants to create a full node without wallet functions.
You are thinking too specific and perhaps PoS was not the best example. But imagine a tiny low-power, low-cost bitcoin node with trivial set-up. I think some people would find innovative uses for these, while others would buy them simply to help support the network.
Mining is a completely moot point. Given the current difficulty, even a million ARM processors wouldn't even be noticed by the network. They would only waste power.
You wouldn't mine with an ARM processor. You would mine with a bitcoin miner that employs 1 or more SHA256 ASICs. If the ARM-core bitcoin node was cheap and simple enough, the "hashing device" could be transformed by default into a "plug-and-play P2P miner" by the manufacturer. This may help to decentralize mining.