Aren't patches made against the kernel GPL'd if the author doesn't
explicitly grant them more liberal BSD license in addition?

That would be the normal assumption.

The problem then comes in taking the patches that were only made
available against GPL code and reshipping them under the BSD license
without the author explicitly agreeing to this.

What if a patch spans both code that is pure GPL and code imported
from BSD, how do you license it?

See the acpi codebase for a worked example.

Alan