Do you mind asking this question in our Platform forum http://forum.piwik.org/list.php?9 ? We usually don't answer support questions here. That'd be great!

Will you add a new setting for this?
The administrator may have a new setting that can be check in order to activate the core function that sets the user id with the remote user in $_SERVER array on every visitor request to piwik http tracker.

I don't think we will add a new setting. It should be already doable with a plugin. You're writing a plugin for Piwik right? Please ask in forum there are two ways to solve this I think. One of it is using our events Tracker.newVisitorInformation (link) and Tracker.existingVisitInformation (link)

Unfortunately, I couldn't get it working with the ldapvisitorinfo plugin. I used these events and sets the uid. I checked the database for any sign of uid successfully sets and I found no sign for that. Maybe it doesn't get saved and therefore the ldapvisitorinfo plugin can't fetch the data.

If it doesn't work there are probably other solutions (creating a VisitDimension, it is not documented yet) but it is most likely supposed to work with events as well. Can you to ask in Forum please? If we identify there is a bug/missing feature we will create an issue here with a concrete example etc.