That’s not quite it, but I finally found it on the SiLabs knowledge base. The power requested by the device is set by a single EEPROM byte, which you can change over USB with an application from SiLabs. What you need is:
It includes some DLLs and other developer tools, but basically you run CP210xSetIDs.exe, select your (already plugged in) adapter by it’s serial number from the device list, check the check box next to Max Power, change the hex value, and click “Program Device.” The hex value is in twos of milliamps, so it comes set to 32 (100ma), and you can set it up to FA (500ma).
There was a note in the knowledge base that old versions of the software let you set the byte all the way up to FF, or 510 milliamps, but that “if this occurs, there is no way to recover the CP210x device, as it will never enumerate on the USB bus.” Yikes! They claim the current version keeps you from doing this, but it didn’t complain when I entered FF into the box, and I didn’t hit program to find out!
-Adam