[galaxy-dev] Any way to prime the value of a tool text field parameter dynamically?

I have a need to have the default value of a text parameter : <param name="statements" label="Statements" type="text" area="true" value="???"> Set dynamically, as powered by say a call to a function in <code file="tool_form.py" /> , much like a dynamic_options="get_rule_section(recipe_file)" call. Has anyone done that somehow without hacking Galaxy code? A user can then edit the text. The default depends on a previous selection (e.g. recipe_file input). Thanks, Damion