FWIW, I personally think this is _absolutely_ the right approach.  
Using a trait is a very visible, very obvious way to say what's going
on that is probably easier to remember than adding another keyword to
the [my|our|temp|let] group.  While I, too, immediately understood what
'has' meant, I can't help but feel many people won't get it.

As others have pointed out, the problem with 'static' is not only that
(a) it has too many C++ meanings, but (b) the word itself implies
'constant', not 'persistent'.  I would really, really like for us to
not use that already-abused word.

These three, I think, show the most promise.  Or the linguistically
dubious "is once", maybe.  The others like "is saved/stored/restored"
might be taken for serialization-style persistence.

I think so... I thought about that too, but I think "persistent" is
becoming synonymous with "serialized & stored" these days.

MikeL