Brief update: The crashes and Valgrind errors when playing back the trace are
almost certainly unrelated to the lockup. It turns out apitrace is too
aggressive in trimming the client-side memory blobs during recording. (See
https://github.com/apitrace/apitrace/issues/407#issuecomment-167866502)

On a more positive note, I am also seeing the lockup on Tonga from inside the
game itself. Unfortunately, I cannot reproduce it reliably yet.