This would be trivial, you can use the swap allocation code (example: see the VN device, dev/vn/vn.c) to reserve, read, and write the swap.

However, I don't see much of a point in doing this. Not everyone configures swap, so you can't count on it, and a system dump will overwrite swap, so you would have to mess around with that as well and I can tell you it just isn't worth the effort. Maintaining an entropy file in /var/db has no downside at all and is a whole lot easier to manage.

This /dev/random stuff is a little wild -- I think the premis is sound, but you really need to look towards implementing more straightforward solutions rather then hacking up unrelated parts of the system. Forget doing special magic in the kernel. Forget using swap. Forget having ridiculously huge entropy files. Simplify it and everyone will be a whole lot happier.