Having some issues with RFID reader
As I said in my original question, I thought about hooking up a lab supply even though, according to the specs, both devices combined fall under the 500mA USB spec.  I put a voltmeter across the 5V bus, and sure enough, it was dropping well below 2.5V anytime I sent a command to the reader.

I hooked up my lab power supply, and immediately my !RW\x0F command returned 12 bytes as expected.  Not the correct 12 bytes, but at least the data was being sent.  It's possible that my reader just doesn't support this "legacy" command.

I then sent the login command, using the default password.  Without a tag, I would receive 0x02, which is "could not find a listen window from the tag".  Makes sense!  I then put my tag over the reader, and unfortunately got a 0x03, which is a "received NAK" message.  Oh well.  I have other things to sort out, but at least I know my mbed code and RFID reader are functioning properly.

Hopefully, this basic debugging technique is helpful to someone.