gistfile1.js

var current_selection = false;
//somewhere in your selection code, set this to 
//true when you have marked a selection
// ...
current_selection = true;
var getKeyCode = function(evt){
  var code;
  if (evt.keyCode) code = evt.keyCode;
  else if (evt.which) code = evt.which;
  return parseInt(code,10);
}
document.observe('keypress',function(evt){
  if(current_selection && getKeyCode(evt) == 32){
    //keep the event from doing anything to your form
    evt.stop();
    $('yourMenu').show();
    //maybe cancel the global variable here, so you can
    //once again type spaces
    current_selection = false;
  }
});