I'm not aware of anything to do that but hopefully someone else will be.
I'm only aware of the method the OP has already dismissed (because it requires a PC to be left on at all times), and that is to monitor the router using SNMP.
Some routers allow SNMP monitoring of traffic on individual Ethernet ports (not sure about the USR, but the Belkin 7633 does this), although this isn't necessarily the in/out bound internet traffic (as it will also count LAN traffic).
Ade
Giving UKFSN/Entanet a try - 1 year and counting!
DL Sync 4000kbps
UL Sync 448