Acquiring a true Hardware overlay under SDL ... How?

Acquiring a true Hardware overlay under SDL … How ?

Seems difficult. 
 
I set my display to 1024x768 and 16 colors (to reduce video memory requirements).
 
I've not aquired a true hardware overlay on any of Win32, Linux, Mac OS X.
 
I do set my primary surface to SW_HWSURFACE.
 
I usually make my overlay a common overlay like YV12 , YUY2.

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