Whoa. Sorry it's taken me so long to get back to you, Paul! Right, you'd use PC Monitor's passphrase/password and the Yubikey as the second factor.
When you touch the button on the Yubikey, it generates the one time password and then types it out to the computer as if it were a USB keyboard. It's actually pretty genius in that regard - it requires no special hardware.
Here, check it out. I'll attach my key and have it type an OTP, then censor the key data:
******######fttrtfdkrerhniclllvnlfiutgdngrrj
The asterisks are a 6 character public ID. The hashes are a 6 character key serial number. The rest is the OTP.