Found a really useful utility which can get the ink level of printer attached to your router. It is called ink, and you can install it like this:
# ipkg install ink
Installing ink (0.5.0-1) to /opt/...
Installing libinklevel (0.8.0-1) to /opt/...
For this you have to have ipkg enabled and properly configured. It will work on your DD-WRT, Oleg or Tomato USB firmwares. Here is what ink tells about my printer jets:
#ink -p usb
ink v0.5.0 (c) 2009 Markus Heinz
EPSON Stylus Office BX300F