Not sure how we missed the documentation tbh, but as it says in the release notes, it needs to be a table, with an index (if it is an array).
This is currently only supported by material properties.
-- set the first vector4 in the array: example[0] = v
go.set(url, "example", vmath.vector4(1,1,1,1), {index=1})
-- set the last vector4 in the array: example[15] = v
go.set(url, "example", vmath.vector4(2,2,2,2), {index=16})
-- set an element of a vector4 in the array: example[0].x = 7
go.set(url, "example.x", 7, {index=1})