Hi,
I have a 95 e320 sedan and the headliner in my car is a hard surface in the front and fabric on the sides and back; the fabric droops. I contacted a headliner person online who answered peoples' questions, and asked if I could use some sort of brad in a couple of places as mine was not drooping all that bad (I have replaced about 3 headliners on other cars and was trying to avoid it). He said that thumbtacks would actually work really well. I actually PAINTED some thumbtacks to match and tried this; they did not even stay in for a second. So, cross that off your list if you were contemplating it. I am still wondering whether you could drill a hole in the hard surface (behind the foam), and place a couple of plastic brads through the fabric and that hole. This could look very jerry rigged but in cases where the liner does not droop too bad I wonder if it might still work. I can only speculate...