I'm not sure why setValue doesn't work here, but then I'm not much of a Moodle developer, so I still don't understand a lot of this. Seems like it should, but it doesn't work for me either.
However, I had a similar question and found a solution that you might find helpful, though you may need to do it a little differently depending on where the form you want to modify is.
First see of you can find a call to the form's set_data() method. For example, I wanted to add default text to the course request form, and found the following line in course/request.php (note that this is where the form is instanciated, not where it is defined!):
$requestform->set_data($data);
Just BEFORE that line, I added something like this:
$data->summary_editor["text"] = "My Custom Text";
That worked for me. Good luck!