I have an old reflector hood I built myself, with a 4" fan on each end that I remember kept even the plexiglass covering from getting very warm (so I count keep it so , problem being that I know the surface above the bulb would likeley get much warmer.. I had originally painted the inside with some high-temp flat white paint, but now that I've advanced a bit and have mylar and 3M spray adhesive around... would mylar or the adhesive likely melt if I cover the inside of the hood with it? a 600W MH,, might want to use it for a 1kw later..
Values vary with manufacturer. A 600w HID gets up to around 400 degrees C. Typical Mylar melts at around 250 degrees C. While it's possible to use enough airflow to keep the Mylar from melting, you'd be safer using a regular textured aluminum reflector. They actually work very well. For DIY hoods and grow room walls, you can make your own with HD foil. Crumple the foil well and then flatten it back out, starting with the edges. This creates an excellent reflective surface with no hot spots.
Stick with white paint or buy a new reflector. What you're thinking about doing is a fire hazard and not worth the risk for an extra gram or two per harvest.