Is it possible to write characters outside the form tag? I know, you can't insert controls there, but is it just possible to insert ordinary ascii characters?
That's not entierly correct. You can indeed insert controls outside the form tag, but only those that cannot cause postbacks. So, a container control like a PlaceHolder you can use outside the form tag, while for instance a button or a textbox must be placed inside a form tag.
If you place this outside the form tag:
You could add controls to it from code behing like this:
myPlaceHolder.Controls.Add(new LiteralControl("My Literal Control