In fact, the first repraps did initially have a servo motor. Last was
Extruder controller 2.2 though makerbot had one version of EC3.6 if i remember ok. The EC 2.2 with some hacks or tweaks documented on site can be made to take step/dir/en signals, and there is a firmware which makes it behave like stepper driver. Thats how ppls started to use stepper motor extruders, which i think was regarded as an improvement. Among other deficiencies, the 2 driver ics didnt had proper protection so they kept getting burned for a lot of ppls. So i suppose one could use EC22 with the board named "magnetic rotary encoder" (also on website), to make a servo loop as it was originally meant. So imho, really not advisable, but hey dont listen to me, you can try if you really want to, and see how it works out. There are some old images for toner transfer to etch your own version if you want to, but keep in mind its a very old thingy.