Belt drive fixed

How much of that 300g saved is lost to half inch thick cogs and chainrings?

Also, can you fix it should it break while out on a long ride?

Having said that, I am all for people investing time and money in good honest tinkering instead of just following the market.

but your chainrings/sprockets can be made of lightweight plastic etc so it would balance out.

In terms of changing the belt, then why not?

Would be far easier to change a dry belt than a dirty greasy chain. Your also more likely to carry a spare as its lighter.

If bike makers used a belt already a standard component in an existing car/motorbike, then it would be extremely easy to get hold of one wherever you are.