Support variable substitution from files/vars generated in initializeCommand

I was using initializeCommand to calculate dynamic values (e.g., finding the real path of a Git worktree, or reading the docker image version/tag from a config file) and storing them in a .env file for Docker Compose to use as variable substitutions.
Now, I have ported all my settings from the Compose file directly into devcontainer.json and removed the Compose file entirely. However, I cannot find a way to run initializeCommand and pass those calculated values back into devcontainer.json for variable substitution (e.g., inside mounts or workspaceMount).