Can a conical form for pouring shorter concrete footers be used for fence posts?
You can't escape physics and violate code, which places footing depths below frost depths.  Just be glad you're not in Alberta, I hear they must go to 12ft.


  
    Where restraint in the form of a building load is present, heaving pressures may or may not overcome the restraint, but they can be very high: 19 tons/sq ft has been measured, and a seven-story reinforced concrete frame building on a raft foundation was observed to heave more than 2 in  
  


Concrete Network article on shallow footings and frost