I have a test case working now, by overlaying a child window with the caption in it.
I did it this way to avoid messing with the CSplitterWnd..CView relationship and to avoid moving GL out of the CView.
Now I just resize the caption when the view changes size, and adjust the top of the GL viewport down accordingly.
Seems to work. Is there likely to be a performance hit on any platform due to the caption window always partially obscuring the GL window? Even then it's not obscuring the GL viewport, so should be fine.
All feels a bit dodgy though. Pity I have to do this! It's not the first Intel-only GL bug I've had to work around.