PlaidCloud expressions and filters provide use of most non-administrative PostGIS methods. PostGIS methods are accessed by prefixing the standard method name with
ST_GeoHash(geometry geom, integer maxchars=full_precision_of_point);
func.ST_GeoHash(geometry geom, integer maxchars=full_precision_of_point)
PostGIS Official Documentation for this method can be found here.
Additional capabilities and usage examples can be found in the PostGIS documentation