I actually changed the Callback definition to include a UINT64 as the third argument, rather than a UINT32.
I can get the gauge to load (I can see the tooltip drawn on the screen) but the gauge itself does not appear and its mouse function is never called.
I suspect that this has to do with the format of the bitmap files themselves, but without any documentation it seems pointless to try to debug this issue.
Using VS2015, not that it should matter unless one is linking against a version dependent lib file.
I can attach the debugger to FSW, thereby discovering that while my custom tooltip is being drawn on the screen, the gauge has not remained in memory.