I see the general issue is in trying to access values that are known but are otherwise inaccessible. Arrays having tuple_size is a good workaround for this specific case, but I'm sure that there are similar issues that have no such solution.
I'd not be in favour of making array::size() static. It seems that it was chosen in order to provide a common container interface, and that's a design consideration that's rather common in the standard library. The justification for that, however annoying, hasn't disappeared.
If it's really necessary I'd have a new static method static_size(), or have the ability to have static methods that provide a static interface and a non-static interface. That seems like a big sledgehammer to crack this particular nut, though.
Honestly I'd sooner see advances to make the more general problem a non-issue (perhaps with the help of [N4433], [P1996], not to mention the smorgasbord of functionality that reflection will give us).