But exiftool still doesn't show the data. I used grep so it's not a matter of looking in the wrong place; although I did eyeball the output too.
exiftool -U -v your.jpg > parsed.txtOpen parsed.txt and scroll down to
| | | 9) CanonCameraInfoPowerShot (SubDirectory) -->
| | | + [BinaryData directory, 592 bytes]
| | | | ISO = 370
| | | | Canon_CameraInfoPowerShot_0x0001 = 411
| | | | Canon_CameraInfoPowerShot_0x0002 = -41
....
That's the data that MakerNote's tag 13 points to; Phil calls it CameraInfoPowerShot. It is a large array of 32-bit integers, the numbers in Canon_CameraInfoPowerShot_XXXX are indices into this array. Inspect entries 91-94 (hex 0x5B-0x5E). In my jpegs, e.g.:
Canon_CameraInfoPowerShot_0x005b = 921
Canon_CameraInfoPowerShot_0x005c = 1469
Canon_CameraInfoPowerShot_0x005d = 1864
Canon_CameraInfoPowerShot_0x005e = 921
Those are Canon WB multipliers for G, R, B, G.
Of course, it is possible that Canon has changed this info or its location. Looks unlikely to me, though. In fact, quite surprising that nobody appears to have investigated it.
Edit: The data appear to have moved a bit in recent cams. In S90, they appear to be at 80-83.