PlaidCloud expressions and filters provide use of most non-administrative PostGIS methods. PostGIS methods are accessed by prefixing the standard method name with
ST_PointInsideCircle(geometry a_point, float center_x, float center_y, float radius);
func.ST_PointInsideCircle(geometry a_point, float center_x, float center_y, float radius)
PostGIS Official Documentation for this method can be found here.
Additional capabilities and usage examples can be found in the PostGIS documentation