AAre you sure you aren't talking about a UPS? I thought by "USV" you were talking about a more specialized UPS that is only meant to safely shutdown a Pc, where a UPS is meant to keep a PC running for a while so you can save your work or until the power comes back on. the thing is I'm not seeing anything for "USV".
A UPS would definitely be too big for this sort of thing, but a shutdown controller connected to the front panel connectors (ie momentary power button) and battery required to simply sustain an immediate shutdown event would not be very big at all. The only question is if there is a product or set of products that will make this possible or if I need to design my own circuit.