There are concerns over using bitumen based products in areas of high heat so this is probably a no no in that respect. Also, silent coat is designed to add mass to panels and change their resonant frequencies. The bonnet panel is unlikely to vibrate due to its shape, size and reinforcement.
The engine bay needs sound proofing rather than deadening (à la Silent Coat) so something like Dodo mat or any acoustic foam would be better suited. This won't stop engine noise coming in the cabin though....