Hi Richard,
I didn't see the episode you mention, but I did watch them do the Elan a few years ago and on that they ground away and seemed to coat it with body filler. It looked great out of the paint shop but I did wonder how long it lasted. I'm afraid I'm old school on this, if I have to take out gel coat cracks I grind away quite a large area into the matting and then build it back up with resin and surfacing tissue. I do use filler, but not so much and mainly for air bubbles that magically appear once I think I've finished.
The place I buy my resin from also sells different grades of filler materials, from the basic powder to different lengths and types of fibre so you can mix your own custom body fillers. It's no cheaper than buying ready-mixed but you do have the option of making it a bit smoother with more resin, which I prefer for filling small defects. Anyway, they sell a short fibre filler for when you need to make a fillet or radius and you need more strength than conventional powder fillers, so if the idea appeals to you, you don't have to stick at carbon fibres. (I'm not convinced the carbon fibre adds anything to this type of application other than expense ! )
Brian