@Michael – thank you for the script.

However, my biggest concern using it is that it will not deal with the document baseline grid settings.
If texts will use this feature one will create overset text frames and/or wrong leading and wrong baseline positions.

To deal with that might mean you have also to take  into account margin properties…

Another concern is: it does not set the app transform preferences. So, if someone has set the property "whenScaling" to "WhenScalingOptions.APPLY_TO_CONTENT" the script might fail. We can change that to "WhenScalingOptions.ADJUST_SCALING_PERCENTAGE":

app.transformPreferences.whenScaling = WhenScalingOptions.ADJUST_SCALING_PERCENTAGE;

Furthermore it does not deal with locked page items. Unlocking a layer does not mean unlocking all page items applied to that layer.

So we need an additional step (see one possible solution):

//UNLOCK LAYERS
var myLayers = myDocument.layers.itemByRange(0, myDocument.layers.length-1);
 
myLayers.locked = false;
 
//ADDITIONALLY unlock all page items with a script menu action (ID = 11396) in one go
//(works on all spreads even master spreads, if all layers are unlocked)
 
try{app.scriptMenuActions.itemByID(11396).invoke()}catch(e){};
 
//A formerly locked object on the actual spread could be selected now.
//So we get rid of that selection to clean things up:
 
app.select(null);

Oh. And if I'm at it (just a minor one):

you could clean up the helper rectangle, if you assign it to a variable and remove the object later on:

var helperRectangle = myPages.item(i).rectangles.add({
    //put your properties here
    });
 
//... do grouping, scaling etc.
 
//Now the helper rectangele is not needed any more:
helperRectangle.remove();

Uwe