I could be wrong, but building an interface between modern tablets and an Amiga doesn't sound /too/ difficult. Are there any current drivers on the Amiga that allow you to use analog paddles in place of a mouse? If so, the absolute-positioning thing is solved. Most cheap active USB tablets are HID compliant (ie: Aiptek), Microchip have some small microcontrollers with USB capability and 12bit ADC, as well as example HID libraries for a few dollars.
Couple that with a couple of I2C/SPI ~500K Digital Potentiometers, and you should be able to throw together a HID to "Analog" converter, which will crap out an X/Y resistance value for an X/Y position on the tablet. I'd have no idea how to action pressure sensitivity though, as I believe the Amiga only has two Analog inputs per port.
I guess if you wanted to sacrifice absolute positioning and opt for relative positioning (like a mouse) to gain pressure sensitivity, you could use the V/H/VQ/HQ pulse inputs to move the cursor, and use the paddle input for pen pressure
...Of course this all assumes an off-the-shelf driver/application exists for this type of behaviour
