I have some concern over using such a system to control existing computer peripherals rather than configuring consumer electronics devices that are not normally hooked up to the computer. It is useful, of course, but presents several challenges.

First of all, established peripherals (printers, UPSs and others you name) tend to have drivers already installed on the computer. There are issues in attempting to communicate with a device which already has a driver talking to it. That driver could of course already provide a means for web sites to talk to the device, which is both good (you are running native code on the machine) but also sometimes more limiting -- you can only do what the driver writers planned for you to do. It also is not portable over all OSs the way raw USB access is.

You would have to suspend normal driver operations. This may not be easy, though in theory should be possible with many devices. In addition, there are security issues with providing access to established peripherals.

On the other hand we don't want most device drivers to be reporting home on us without our knowledge or consent. A key factor of the plugin I describe is that it would be an overt act. Go to a web page with the plugin, and see a dialog pop up saying "Do you want to give control of USB device XXX to the YYY web site?" Letting the power company monitor a UPS is more spyware-like.