We currently have a java client which performs coordinates transformations via the Geometry Services SOAP client, but a client has requested we use the REST api instead.
With the SOAP interface we have used JAX libraries to generate java stubs and this lets us specify the following parameters:
InSpatialReference
OutSpatialReference
TransformForward
Transformation
Extent
InGeometryArray
Where Transformation is a GeoTransformation WKID or WKT object.