Thanks for your feedback, folks. I presumed that <link rel="import"> is one-shot just because it is how <element> works and I felt both are analogous, but apparently this is not a common perception.
It seems like making <link rel="import"> dynamically-updatable isn't that controversial. I'll try that way. Filed https://www.w3.org/Bugs/Public/show_bug.cgi?id=20683
