This is a feature proposal for the following use case:
Let's have two seperate Redmine installations, one being held in a DMZ allowing customers from the internet to access the system. In the Intranet there is a second installation which holds the "sensitive" data, e.g. which shall not be disclosed even if Redmine's security measures break.

Due to the last restriction a per-project user restriction would be a solution but not the "political correct" one - which is requested in my case.

The idea is to allow the internal Redmine installation to link to tickets in the external/DMZ installation, i.e. handling a bug/feature in the internal Redmine and pushing mid-time status reports to the customer(s) in the external Redmine installation.

I'm willing to further explain this idea and/or do a proof-of-concept implementation although my Ruby (and/or Rails) skills are very poor (haven't had the time to learn the syntax and concepts yet).