@Jamie, I got excited, dug in, and solved the problem in the latest commit by adding a new --multiline-json flag.
@bschmitt If you want to give it a try, you can apply the patch and see how it works for you:
webmin patch https://github.com/virtualmin/virtualmin-gpl/commit/475dfc8
After further code improvements, we updated the parameters to output the same format as the remote API. The parameter names were also changed to --json and --xml for the CLI.