On a windows machine, authenticated on an NT domain, the proxy username and password appear to be sufficient for getting the lightweight http wagon to download artifacts.
On a linux machine, which is connected to the domain's network but not authenticated on the domain, the same settings do not work.
To further compound the confusion of the situation,
setting http_proxy with the specified username and password using
allows me to use wget on an artifact's URL and retrieve it from the command line, indicating an issue with the proxy settings for java (since a compiled app from the command line successfully navigates the proxy).
There appears to be no additional logging available.
Attached is the output of mvn -X that shows the trace.
Currently checking out the trunk sources to attempt further debugging.