PlaidCloud expressions and filters provide use of most non-administrative PostGIS methods. PostGIS methods are accessed by prefixing the standard method name with
ST_LineInterpolatePoint(geometry a_linestring, float8 a_fraction);
func.ST_LineInterpolatePoint(geometry a_linestring, float8 a_fraction)
PostGIS Official Documentation for this method can be found here.
Additional capabilities and usage examples can be found in the PostGIS documentation.