@JasonHarper delivered the simple answer.

encoder = RotaryEncoder(a=14, b=15, max_steps=1000)
encoder.value=.5

value and max_steps are both described as properties in the documentation. max_steps can be an argument. value needs a separate assignment as above. The typography on the man page is unclear. Despite being a property, max_steps cannot be defined outside RotaryEncoder; there is no setter method.