I have only done a little bit of glassing but I will give you my take on it.
I would make a frame work out of MDF then, you can use spandex/ponte/fleece to "skin" the dash after which you would reinforce it with fiber mat. As far as heat goes in my experience it's not a huge deal if you don't rush things. Lay up 3-4 layers and let it cure,then you can decide if you wanna change the shape here, or there. Another possible method would be make your frame work out of ??? MDF if it was me,then use potters foam (the green stuff at walmart) and build up and shape how you want it. The resin will not eat it,and its not very effected by the heat buildup. Have you visited http://www.fiberglassforums.com/
? If not I would suggest it as there are alot of dash builds going on there.