Since I was unable to find " = { 0 }" anywhere else in lengthy Google searches of Arduino and C websites, I get the feeling this may be an "uncommon" use of this syntax
It would be reasonably common. You need to search in the context of initializing arrays. Otherwise you find a lot of stuff about soccer scores.
Whilst Coding Badly is correct that static variables are automatically set to zero, automatic variables aren't*. So this could be a reasonable way of initializing an array in a function:
void foo ()
{
int buffer_pos[NUM_AXES] = { 0 }; // start with all zero
// blah blah
}
*Edit: He didn't imply they were.