11-202v3 AETC Sup:
8.18. IFR "VFR-on-Top. " Unless otherwise restricted by the MAJCOM, PICs may request
and fly IFR "VFR on Top" operations (IAW AFMAN 11-217V2 guidance) when the mission
requires such clearances. If the PIC cancels the IFR clearance, comply with Chapter 7.
8.18. (AETC) IFR ―VFR on Top. ― VFR on top operations are prohibited unless prescribed
in AFI 11-2MDS-specific or syllabus guidance.
Also:
7.3. Flight Operations under VFR. <snip> PICs of VFR aircraft must at all
times be able to control the aircraft by visual reference to a discernible horizon, regardless of
cloud clearance requirements.
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It is specifically prohibited by AETC. Other MAJCOMs can make their own rules. Personally, I'm not a fan of the VFR on top plan of action because of 7.3. Sloping cloud decks often give a false horizon and if you've every flown while looking at one, it's very disorienting. Also, the plane I fly hardly ever is below FL180 anyway so it's a not really an option anyway.