Tim,
Standard resin will start to break down after 90deg Centigrade. Pre preg is heated to cure and while much of the curing can be down at 80 -100 deg C it needs a period up around 110-120 deg C to make it resistant to high tmperatures after it has been cured.

I can't tell you exactly how much weight you would add by painting a Mast. But I have painted beams once. It adds around 1kg per beam. Notice I said once.
I couldn't get over how much weight it added. Now given the prep material before the top coat adds weight and prep material will most likely not be required on the epoxy mast I'd guess you would still add close to 1kg to its weight and that is a weight increase of around 10%.

Personally I wouldn't paint it.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!