I'm Gianluca Caporello the main developer of Seats-Tramo, by now, at the bank of Spain we developed a new release of ST which export a C++,Java and Python (and a primitive R) API to ST (Windows and Linux).The API is not documented but it's running from some years on a FAME integration and we have a lot of running examples.

If interested for a better integration in GRETL feel free to contact me or give a look at http://www.bde.es/webbde/es/secciones/servicio/software/econom.html