I'm not sure I understand you correctly, but.

Assign the browse backward to the back hardbutton.

then:
var inactivity = true;

page().onEntry = function () {
scheduleAfter(30 * 1000, checkInactivity);
}

function checkInactivity() {
if (inactivity) CF.widget(PS_BACK).scheduleActions();
}

This will start a wait of 30s after the page is loaded. If nothing happens during these 30 secs. the the remote will go to the previous page. I don't know what you do with that button, but if you add:

thebutton.onPress = function () {
whatever it has to do;
inactivity = false;
}

Then if you execute the keypress, the remote won't browse backward any more.