I have made a start with support for OpenDDS but I found out that ROS2 does some tricky things with DDS Guids and InstanceHandle types which are outside of the DDS specification. Adding OpenDDS needs some more structural work but the work done until now is online at https://github.com/jwillemsen/rmw_opendds and https://github.com/jwillemsen/rosidl_typesupport_opendds.