After thinking about Morphing [Interactive] and Droste filter, I believe that the better design for the Droste is the one with interactive. That being said, is there some documentation to develop interactive filters?
Basically, you need to master the use of the window command, as well as how user events can be caught in G’MIC (see the part of the Substitution rules dedicated to the display windows).
Also command wait is useful here.
Beware, this is not the simplest kind of things we can code in G’MIC scripting 
You should start looking at the code of simple interactive commands, e.g. x_cut.