rosbag2 mcap plugin is now available for galactic:
$ sudo apt install ros-galactic-rosbag2-storage-mcap
$ ros2 bag record -s mcap -a
Foxy & Humble builds should be out in the next couple of weeks (whenever the next Packaging and Release Management - ROS Discourse is). Please direct feedback, feature requests, and bug reports to GitHub - foxglove/mcap: MCAP is a modular, performant, and serialization-agnostic container file format for pub/sub messages, primarily intended for use in robotics applications.