It is the functionality of DDS_InstanceHandle_to_GUID, that is not something which comes from the DDS specification. I haven’t found a way to do this with OpenDDS (there DDS::InstanceHandle_t is a long) and I need to dive much more in the details of ROS2 to see if this can be done in a different (and spec compliant) way. I reported this as issue https://github.com/ros2/rmw_connext/issues/326.
As part of my work I already made several pull requests to rmw_connext, these are all integrated in that RMW implementation, thanks for accepting my proposed changes.