I had to add a simple filtering by site type id (without this it will be a mess with 2 or 3 websites on one ss). It's not perfect but maybe someone will find it useful or will polish it further... This is how I did it.

1. subsites/javascript/LeftAndMain_Subsite.js

Modify onchange with the following code:

onchange: function() {
			if($('Form_AddPageOptionsForm_SubsiteID')) {
				$('Form_AddPageOptionsForm_SubsiteID').value = this.value;
			}
			var subsiteID = this.value;
			jQuery.ajax({
				  url: SiteTreeHandlers.controller_url + '/changesubsite?SubsiteID=' + this.value + '&ajax=1',
				  success: function(sitetree_res) {
						jQuery.ajax({
							  url: SiteTreeHandlers.controller_url + '/AddPageOptionsForm&subsiteID='+subsiteID+'&ajax=1',
							  success: function(responseText) {
									// Modify form with ajax response (we take only options, the reset remains unchanged) 
									var cur = jQuery('#Form_AddPageOptionsForm').attr('id','Form_AddPageOptionsForm_Temp');
									var cur_inner = jQuery('#Form_AddPageOptionsForm_PageType').attr('id','Form_AddPageOptionsForm_PageType_Temp');
									var mod = jQuery('<div style="display:none">'+responseText+'</div>').insertAfter("#Form_AddPageOptionsForm_Temp");
									var mod_inner = jQuery("#Form_AddPageOptionsForm_PageType").html();																		
									mod.remove();
									cur_inner.html(mod_inner);
									cur.attr('id','Form_AddPageOptionsForm');
									cur_inner.attr('id','Form_AddPageOptionsForm_PageType');
									
									// Fix for CMSMainLeft
									addpageclass.prototype.originalValues.length = 0;									
									jQuery("#Form_AddPageOptionsForm_PageType option").each(function(){	
										addpageclass.prototype.originalValues.push({
											'value': jQuery(this).val(),
											'label': jQuery(this).html()
										});
									})
									
									if ($('sitetree')) {
										$('sitetree').innerHTML = sitetree_res;
										SiteTree.applyTo($('sitetree'));
										$('sitetree').getTreeNodeByIdx(0).onselect();
										$('siteTreeFilterList').reapplyIfNeeded();
									}
							  	
							  }
							});
			}});			
		}
	},

2. cms/code/CMSMain.php

Modify AddPageOptionsForm() with the following code at the end:

		$out = new Form($this, "AddPageOptionsForm", $fields, $actions);
		if(Director::is_ajax() || $_GET["ajaxDebug"])
			return($out->forAjaxTemplate());
		return($out);

3. sapphire/core/SiteTree.php

In page_type_classes() at the following code:

			$current_subsite_id = Session::get('SubsiteID');
			$parent_subsite_id = $instance->stat('parent_subsite_id');
			if(isset($_GET['subsiteID']))$current_subsite_id=(int)$_GET['subsiteID'];
			if(isset($current_subsite_id) && isset($parent_subsite_id) && $parent_subsite_id != $current_subsite_id) {
				$kill_ancestors[] = $class;	
			}

after:

			if($ancestor_to_hide = $instance->stat('hide_ancestor')) {
				// note for killing later
				$kill_ancestors[] = $ancestor_to_hide;
			}

4. In all Pages where you want to limit page visibility to just one subsite add the following code to your class:

public static $parent_subsite_id = YOUR_SUBSITE_ID; //your main site has "0" id

Without parent_subsite_id your page will be visible for all subsites

I tested this hack on SS2.4rc1 and it worked for: Create Page, Behavior tab and even with allowed_children set up.
It's not perfect solution but I had to make it quickly since we I need to add a few subsites very soon and I have to move on to other tasks.