Using an external simulation clock

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Is it possible to use Gazebo with an external simulation clock source? E.g. have use_sim_time:=true but have the /clock topic published by another process? I'm using ROS:kinetic w/ Gazebo7

I'm trying to glue together some non-ROS programs with Gazebo. One of these monolithic beasts provides a sim clock over UDP and it'd be much easier to write some middleware to pipe it into /clockto set the pace instead of adapting it to use any real of ROS in the short-term (eventually we'd move in that direction).