Caching just the username for authn

Somebody approached me a few minutes ago at ApacheCon and posed the
following question:
"The username on my local box is different than my username on the server,
and I don't want to cache passwords. How can I cache just the username?"
 It turns out I didn't have a good answer for him, aside from the ability
to put '--username $FOO' on the command line. Sander looked into the
config file and it turns out we don't have a configuration option which
users can set, either in a global context, or on a per-server scenario.

Before I run and file a bug on this, I'd like to make sure I'm not missing
something terribly obvious. Am I?

-Hyrum

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