shows the pinouts for usb
I think the power pins are set in stone; but with a usb "crossover" (as I will call it for simplicity) which is what used to be called a null modem cable, obviously the two data pins can be switched (at one end).
As Jeff comments there were (still are) also adapters (rs232) which did cross over for you to allow you to use standard cables between each device and the cross over adapter; and similarly for parallel port cables (db25).
http://ccm.net/contents/197-linking... shows those pinout (out of interest...)
There are usb modules which allow you to check with a volt meter what's on which pin - at least the voltage pins... One of those would allow you to check on the modem/router what is what; if there is 5volts on the correct pins of the modem router - and on the tablet equivalent pins... then I would be inclined NOT to try connecting them directly...
There also usb plug in modules which display the volts for you; and also - on some - other info as well.
However after rereading your information and having gone through the above research/cogitations... I note that the modem/router is either charged by a charger or a computer. If the tablet is similarly designed... and doesn't produce 5volts usb power then you can't do what you suggest as with the modem/router and computer?
I would secure/get a usb voltage tester and see what each device puts out in terms of volts on the power pins. That way you will know if there are volts on the tablet; and also the modem/router.
Incidentally what is the power in/charger connector style for the modem/router; and similarly your tablet?