Not enough info - give code example! I have not tried but should be as simple as:
v2dfi.keyValue = [ [318.0, 8.0], [656.0, 8.0], [318.0, 8.0] ]
Who is returning you false, .setField("keyValue", val)? If so, my bet would be your `val` is wrong type.
I noticed SG gives no error on wrong type... Roku tends to be like that, silent if deadly