In RS world exist only 2 lines. RX (listen) and TX (speak). You can connect any two devices and they will talk to each other. Devices may use different languages and understand only certain interface but at least they can notice 'hey, somebody talking to me'.
In USBw world You have host (master), device (slave) and single channel (bidirectional twisted pair). Host can command (transmission to device) or ask every 1ms (device reply - transmission to host). The problem is that You cannot connect two hosts or two devices together.
PC, RaspberryPI, some mobile devices and few development boards have USB host. Your tablet, Arduino, Teensy and a lot of other stuff have USB device.
The WaxBee converter read data from serial/ADB tablet and emulate USB tablet.
Yes, You can buy add-on board for Arduino that have USB host, but it's very expensive and complicated. So I never play with it.
Apart from that, there are at least two more solutions.
1. Get computer (rPi should do it).
2. Buy tablet with serial interface, like this one: http://www.ebay.co.uk/itm/Wacom-Digitizer-II-UD-1212-R-12-RS-232-Serial-Interface-tablet-only-/201725222515?hash=item2ef7c29e73:g:e0IAAOSwj0NUcObw
or spare screen with digitizer from almost any tablet-PC like this one: http://www.ebay.co.uk/itm/Fujitsu-T-lifebook-Wacom-Digitizer-drawing-Tablet-12-inch-Art-digital-paint-/252207720461?hash=item3ab8c0680d:m:mwTI6iHyrV5-V1R8vVPU5tg