If one wanted to DIY their own continous rotation servo, there is some possibility of having position feedback. The pots in servos I've modified don't have mechanical stops on the pots themselves. If one can work with small circuit mods. one might be able to electrically disconnect the internal pot, substitute a small resistor voltage divider for the internal pot, then externally read the origional internal pot.
That is possible. Remove the wiper wire and wire it to the center of two 10k ohm series resistors wired to Vcc and ground to make the servo electronics happy and use the pot's wiper terminal as a feedback signal to an arduino analog input pin. Keep in mind that the pot will have a 'bad spot' that will read flaky most likely, were it wraps around from min to max on the pot.