i was also gonna mention the same as the above poster... i think it would work pretty good, so long as the tarp was plenty thick and didn't allow anykind of light in or out... after all, what are all of these grow tents that they sell? i have seen one at my local hydro store, and although it is not a tarp per se, its not that far from it..
i would just go with the thickest tarp material that you could find... also, the idea of lining the inside with mylar sounds good too, as this would help with not only reflecting your light back to the plants, but it would help to keep any unwanted light out of the grow..
instead of vegging and flowering under one tarp, wouldn't just using to seperate areas, one for each stage work as well.. and if you use 24 hours of light for the veg like a lot of people do, you wouldn't have to worry as much about light leakage issues..
best of luck with whatever you end up going with... grow on...