I know nothing about metalcast, but my brother knows his paints better than anyone, and alot of his knowledge has rubbed off on to me in recent months.
In my experience, shellac sticks to nearly everything, and nearly everything sticks to shellac.(what marvelous little bugs!)
Most products I've used call for an industrial epoxy basecoat, if you use it, you'll have no trouble, it's rather expensive though, and I doubt necessary with metalcast. The fact metalcast comes in aerosols tells me it's nothing too fancy. This is New Zealand after all
Basically, use what the can says to use, because in the long run, the finish won't always last too long otherwise.
Now, if it were me finishing this guitar, and I wanted a translucent red, I'd tint shellac with universal tinter(ask your local paint store for some) when you have a satisfactory colour, I'd use cabbot's CFP, it brushes well, is rock hard(designed for floors) and takes 3 days to harden. I think you can get a 1litre can for about $25NZ at most hardware stores, a 4 litre has done my work benches(which are huge btw), a guitar, and several other things, and I've probably still got 3.5 litres left. Don't bother with the optional hardening additive if you get some, the stuff straight from the can is plenty hard enough for a guitar. The last guitar I built had about 5-6 coats of CFP over tinted shellac, and that came out awesome, not factory standards, but I didn't buff it. just levelled between coats.
Edit: Had a quick read up on that metalcast stuff, without the right primer, you'd be wasting your time, it would probably bubble after a few weeks.