DECam has its rotator angle set to NaN in astro_metadata_translator, and its rotator type to UNKNOWN. Since it is on an equatorial telescope, we can just pre-compute the correct value, set rotType=SKY and set those in the translator and they will be correct for everything (or at least correct enough to provide an initial guess).
The diagram on this page should help figure out the correct on-sky rotation angle.
One open question is whether we need to set a rotatorHandedness (when +Y is north, is +X is east or west?) value in VisitInfo, or whether we can easily get that from the camera geometry. At present, VisitInfo does not preserve this information, because Russell Owen assumed it would be managed by the camera geometry.