Warming PCB in a low temperature environment
I would be inclined to use traces on or in the PCB as a direct heater, as you suggest. Obviously, you'd start by insulating the board as much as possible to minimize the energy required to maintain temperature. Pay particular attention to external electrical connections, which can also be good conductors of heat. Extra lengths of wire buried in the insulation will help quite a bit.

I would put the trace(s) mostly around the edge of the board, generally keeping them away from other heat-producing components in order to keep temperature gradients across the board to a minimum. I would design a current source that has temperature feedback to drive the trace, with a "maximum" setting that's guaranteed not to burn it out.

The flexi heaters you found are essentially doing the same thing, but they probably consider the applications data somewhat proprietary. You'll have to work out the details yourself; things like trace widths and thicknesses, current levels and temperature rise, based on the basic resistivity of copper, which is 1.68&times;10-8 &Omega;-m @ 20&deg;C.