One way I was taught in the uk armed forces (a very long time ago) was to use dry rocks.
Heat the rock in your camp fire, then carefully just before bed time, place the rock inside your shelter, the rock releases it's heat slowly and acts very much like a radiator producing a consistent source of dry heat.
A word of warning though
this method was never designed with polyester tents in mind, it was designed more for the hand made shelters that you may build in a survival situation, and where the heated rocks would be placed on bare earth.
With a little bit of common sense however, for example place the hot rock on some logs or a layer of thick sticks, and not allowing it to touch portions of the tent that prolonged contact with t may melt, then you should be pretty much ok.
The heat it will generate will be dry dissipated heat such as would be obtained from a radiator or convection heater, so should not in any way damage the tent, also make sure that you heat rocks that are 100% dry, if there is any moisture at all in them then you run the risk of them cracking and even exploding in the fire, it's not funny when it happens either, I know from experience!!
It's not a perfect solution, and it does take you a few hours to heat the rocks up, then you also need a suitable way to carry them, because they WILL burn flesh quite easily, but if done with care, it will work and should work well.
Update - 23/9/2014
I was just re-reading this, when I remembered something else that's of use in this situation.
Many of you who do outdoor type stuff might carry with your supplies some of those reflective Mylar survival blankets (Like the type rescuers use when they rescue people who are hypothermic).
Whats often not realized though is these things are not just good for wrapping yourself in, but, if you use a bit of duct tape fashioned into a loop you can stick them up on the inside of your tent above your head and up the sides.
Beacuse of the reflective properties a lot of the heat that rises from things like body heat, or other heat sources in the tent, gets immediately reflected back down over back into the main tent space.
Also because of the general thermal properties of Mylar, any heat that does make it through will be a lot less than any heat that would escape directly through the polyester/canvas on it's own.