Passing Matrices as params in Launch Files

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I'd like to pass the data contained within a matrix as a parameter to several nodes in a launch file. This link states how to pass in std::vector<...> params for bools, ints, doubles, etc. Is it possible to pass in something like a std::vector< std::vector<int> > via the launch file?

I'm looking for something similar to this answer: given a launch file

<launch>
<node name="..." ...>
     <rosparam param="matrix">
         [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
     </rosparam>
</node>
</launch>

I'd like to recover the value of matrix within the node by calling something similar to:

std::vector<std::vector<int> > matrix;
nh.getParam("matrix", matrix);

This method didn't work when I tried it though. Is there another way this can be done?

(I'm using ROS Kinetic on Ubuntu 16.04)