The Mr. Boom core should work now with netplay! Use the latest RetroArch in order to do multiplayer online!
To get Mr. Boom, in RetroArch go to Online Updater -> Update Cores. Select 'Mr. Boom' in order to download it.
From there you can start Mr. Boom by going to 'Load Core', select 'Mr. Boom'. Then select 'Start Core'.