Hmm... I actually did a quick check with the keyboard - no dice. Just the one red LED glowing relentlessly.
BTW, the output from dfu-util when downloading was:
dfu-util 0.8
dfu-util: Invalid DFU suffix signature
dfu-util: A valid DFU suffix will be required in a future dfu-util release!!!
Opening DFU capable USB device...
ID 0483:df11
Run-time device DFU version 011a
Claiming USB DFU Interface...
Setting Alternate Setting #0 ...
Determining device status: state = dfuERROR, status = 10
dfuERROR, clearing status
Determining device status: state = dfuIDLE, status = 0
dfuIDLE, continuing
DFU mode device DFU version 011a
DfuSe interface name: "Internal Flash "
Downloading to address = 0x08000000, size = 262080
Download [=========================] 100% 262080 bytes
Download done.
File downloaded successfully
I'll try another keyboard tomorrow, too.