FYI: I am using radenshield under the tiles (make sure you have venting for the hot air - monier is able to help there!) + cellulose insulation on top of plasterceiling like you plan do to. No problem with the weight.

This in combination keeps the plaster ceiling at ever so sightly above ambient temperature even on the hottest of days. Never is it more than 2° degree above ambient and with proper ventilation you wont trap heat in the roof during the day.

What you thought of is probably ideal for Malaysia if you consider ventilation for the roof as well.