FSR wiring using internal pullup resistors?
There is probably no practical difference concerning high side vs low side (except the Arduino Uno only has a pullup, and no pulldown).

The issue would be the accuracy of the pullup (graphs in the datasheet show variations of 10% with temperature) vs the accuracy of the resistor (1% is now commonplace).

The pullup is usually a current source, not a specific resistor value. Even if the supply voltage is not known, the maths involved with solving a known resistor is series with an unknown resistor is quite straightforward.

Short answer, using the pullups would probably work, but you would have to confirm accuracy, calibration and even repeatability, while this is already well known for a resistor.

Datasheet, see graphs on p412:

http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-42735-8-bit-AVR-Microcontroller-ATmega328-328P_Datasheet.pdf