Reproject to Non-standard Coordinate Systems

Hello, I would like to use `reproject_interp` to reproject a FITS image in the Galactic coordinate system to a non-standard FITS coordinate system (such as the Magellanic coordinates or the Sagittarius stream coordinates). How can I achieve this?

Start with the reproject_interp entry point and its existing FITS-coordinate handling. Determine whether non-standard systems such as Magellanic or Sagittarius stream coordinates are supported, and clarify whether the outcome should be documentation or a new capability. Done means a reproducible path for the stated reprojection, with guidance or coverage for the non-standard system.

Written by the indexing model from the issue text.