Yes, I was finally put in contact with NVIDIA Engineers who reviewed the above code sample (fbo_sample.cxx) and asked why we were making calls to glXSwapbuffers() when we’re using framebuffer objects. They suggested using a simple glFlush() before mapping the FBO to the screen. This resolved the issue - no longer enabling ‘Idle’ mode when the screen is locked.
Note: the driver was detecting calls like glXSwapbuffers() and assuming the X Application did not need to run when the screen is locked, thereby enabling ‘Idle’ (low power) mode.