Well I just found the problem.

When I took the tube out I found a smallish hole, which I patched. I
then reinstalled the tube, pumped it up to 100psi, and it promptly
burst again.

Suspecting a dodgy patching, I took the tube out and did a bubble
test, only to find that the patch was intact and not leaking. Then I
found another hole, but interestingly enough it was located exactly
opposite the valve from the previous hole. Although inspection of the
tyre hadn't shown any issues, I now had an idea where to concentrate
my search. (I should add that I have the writing on the tyre lined up
with the valve, hence the relative position of the tyre to the valve
is constant, though evidently I'd turned the tube around when I
reinstalled it.

Sure enough, there is a small hole in the sidewall right near the bead
of the tyre.

At the time of the burst today I was riding on smooth and relatively
clean bitumen.

So I can't blame the tube, it looks like I might have a dodgy tyre
after all. I'll see if Torpedo7 is willing to send me a new one or a
refund but in the mean time will install a different (and
coincidentally wider, since that's all I have) tyre on the back.

Travis