It is possible - at least, if you want to display text, allthough it doesn't seem to be too well documented. In your print composer template add a label as a placeholder for each parameterized text and make sure, that you enter a unique Id for the label.This id is the name of the GET or POST parameter to be used in yout GetPrint call.
You can even make your webclient responsive to the id-"alized" labels in your composer templates, because the ids of labels are listed in the ComposerTemplates section of the response of GetProjectSettings-request, like:
<ComposerTemplate width="100" height="200" name="myTemplate">
<ComposerMap width="90" height="190" name="map0"/>
myLabel2 are the ids of your composer labels.