This certainly looks very trivial, but after reading (I admit, it was more like skimming) the prototype.js documentation, I couldn't find a way of doing this. Of course it can be done just by using plain Javascript, but I really wanted to take advantage of the Prototype "abstraction" layer and avoid any possible cross-browser issues.

The solution came from a similar example, changing the attribute of an element:

  var form_element = $('my_input_text_field');
  form_element["value"] = "new value";

A very straight-forward one-liner, especially if you know about it. Prototype.js never ceases to surprise me!