The CQL tutorial page contains an example showing how a polygon can be used in a DISJOINT query:
DISJOINT(the_geom, POLYGON((-90 40, -90 45, -60 45, -60 40, -90 40))
What is the syntax for specifying the coordinate reference system of the
POLYGON object's coordinates?
The assumption is that the polygon's coordinates are in the same CRS as the layer being queried, but I'm hoping that if the polygon's CRS can be specified, GeoServer may be able to perform the calculation on-the-fly and return a meaningful result regardless of the CRS of the layer. (Please let me know if this isn't going to work, and I need to re-project the polygon before performing the query.)
The answer at https://gis.stackexchange.com/a/29630/3112 says
call setSRS(POLYGON(...), 'EPSG:xywz')
but I can't get
setSRS to work, and I can't see that listed anywhere else.