As of DM-14932 it is now possible to construct a celestial WCS from camera geometry and the telescope pointing. The new function is
makeSkyWcs(pixelsToFieldAngle, orientation, flipX, boresight, projection="TAN")
The resulting WCS will have a CRVAL equal to boresight and a CRPIX at the center of the focal plane (thus well off most CCDs in a mosaic camera). The orientation is of the focal plane, not detector pixels, which has the benefit that you can use the same value of orientation for all detectors, even if some detectors are rotated with respect to others (e.g. HSC). See the source code documentation for details.