Sure.  I also wonder if there are any security implications here.  Maybe "Don't open fits files with GalSim from people you don't trust."  :)

Probably the most important caveat is that it might not work to write the fits file on one system and open it on another.  It may produce a garbage wcs after the round trip.  Or it may fail (raise an exception), in which case the galsim.fits.read function will move on and probably end up with an AffineTransform.