Here’s a bit of a more complete answer. This is totally unsupported and undocumented; it seems to work as of 1.0.0, but may break at any time.
If you run daml sandbox --unknown-flag, the sandbox will fail to start and complain about the flag not being known (for obvious reasons). In failing to start, it will show the full command that was run, in my case:
$ daml sandbox --unknown-flag
Error: Unknown option --unknown-flag
Try --help for more information.
daml-helper: Received ExitFailure 1 when running
Raw command: java -Dlogback.configurationFile=/Users/garyverhaegen/.daml/sdk/1.0.0/daml-sdk/sandbox-logback.xml -jar /Users/garyverhaegen/.daml/sdk/1.0.0/daml-sdk/daml-sdk.jar sandbox --unknown-flag
This shows you the raw command, in which you can see how to pass a different logback.xml configuration.