Recently I was working with a customer and they wanted to log the user that launched the iFlow. This was unfortunately far from obvious and is not something that any of us have seen before.
A quick call to engineering led us to the SapAuthenticatedUserName header value. Provided that the user is being authenticated by Basic Authentication it will be provided in the message header where it can be consumed in your iFlow.
Here is an example that is setting the body on a message with a user name value.
It is also important to remember that the header needs to be allowed for the iFlow so in the runtime configuration of the iFlow the header needs to be added to the allowed headers list.
Of course we have documented all of this:
Hopefully this will prove useful for someone