If Toho actually does consider Zone Fighter to be part of the Showa Godzilla timeline, then it is canon.
Personally I prefer Zone Fighter to be canon to the Showa Godzilla continuity for all the good feats it gives Showa G & Ghidorah, as well as closure on what happened to Gigan.
The wiki page does have a Sci-fi japan article link from November 14th 2009, in which it reads that Zone "reportedly" takes place between Godzilla vs Megalon and Godzilla vs MechaGodzilla.
Wikizilla's page links to the same article:http://www.scifijapan.com/articles/2009 ... ies-guide/
I'm inclined to believe Zone is part of the Showa timeline, since it doesn't really contradict anything established before, and adds to the Showa series by:
* Giving Showa Godzilla the strength to throw 80,000+ ton monsters, as apposed to only 40k ton monsters in the films alone.
* Showing King Ghidorah holding his own against a 55,000 ton Zone fighter, evidence that contradicts the notion that Showa Ghiddy is weak.
* Giving us the closest we may ever get to an Ultraman & Godzilla crossover.
* OG Gigan's demise.
* Zone fighter's interesting powers like the meteor missiles and teleporting through a TV broadcast.