We do not have a direct policy regarding this. Long story short: work created at the university is owned by university unless declared prior to start of coding.
I have discussed with my boss from time to time about this and we have not gone far with it as it becomes a political nightmare as the people who need to approve this do not quite understand open source software aside from the "it is usually free" idea.
/me eagerly awaits others responses!
Update: This topic surfaced at work recently, we met with our legal team and all appears well. My understanding (from my employer) is this is a case by case basis but they are willing to work with employee's interested.