- load your own javascript file with something like:

<?xml version="1.0" encoding="UTF-8"?>
<openerp> 
    <data>
        <template id="assets_backend" name="mis_builder" inherit_id="web.assets_backend"> 
            <xpath expr="." position="inside">         
                <script type="text/javascript" src="/path_to_your_js.js">
                </script>     
             </xpath>    
         </template>   
     </data>
</openerp>

Then in that javascript file, do something like

var map = {x: false, y: false, z: false};

where X, Y and Z are the value of the keypress event of the key you want the user to press (for exemple, "enter" is 13)

then something 

$(document).keydown(function(e) {
     if (e.keyCode in map) { 
         map[e.keyCode] = true;     
         if (map[x] && map[y] && map[z]) {    
             //code to execute when X, Y and Z are pressed. Again, X Y and Z are the value of the keypress event
}).keyup(function(e) {
     if (e.keyCode in map) {   
         map[e.keyCode] = false;  
}});

The code to execute should, in some way, hide or display the field you want. You'll need to find a solution to use a CSS selector to find that field exactly, but that should not be hard ;).