Bootcamp: Windows installer couldn't find drivers. I placed them on the installer USB. Where should they go?
Just following up. I unmounted the .ISO file, deleted the windows partition and re-created the USB drive... Seems to have worked this time. Gonna chalk the issue up to an anomaly.

Like @DavidAnderson pointed out in the question comments, users with a Retina MacBook Pro 15 inch should use This version of the support software, not the one I linked in the original question.

Cheers.