Project Server Site (PWA) cannot be edited using SharePoint Designer

While developing on the Project Server (PWA) , I got a requirement to change some design (UI) of the site. Therefor I tried opening the site with the SharePoint Designer (SPD). But to my surprise I got the following message.

After doing some research I learnt that, on the site definition level, Microsoft disabled the editing in SPD. On the "ONET.xml" file the following key does the disabling act.







Now,the tricky part is if you remove this key on the site definition file, there is a huge possibility that your site might break and Microsoft does not recommend this modification on the ONET.xml file.

DisableWebDesignFeatures="wdfopensite"

This discussion thread has some more details.