How to formalise the "extra-mile" work done by an academic? In my experience, academics are almost always expected to contribute some of their time to activities beyond their principal teaching or research roles. These extra tasks include, for example, attending open days, visit days, serving on one or more committees, acting as head of a student year group, admissions officer - the list is long. 

Whether this should be the case is not in question here. 

In one of my previous institutions, there was in place a "brownie-point" system which was supposed to keep a track of how much extra administrative/organisational/outreach or otherwise "extra-mile" work an academic had taken on.  When a new task required action, the academics could use their accumulated points to argue why they shouldn't (or indeed should, in some cases) be allocated the task. Setting the value of a task relative to all the others, as you might imagine, raised some difficulties.

My question is: has anyone experience of any other kind of formalised system of evaluating and allocating these "extra-mile" tasks?
At my university, we have this system for PhD students.

I am a PhD student. In my contract, I have "up to 20% department duties". When I get assigned tasks not related to my research — teaching, administrative tasks, presenting our institute to visitors, etc. — I write down the hours. At the end of the semester, I report to my boss how many hours I have worked on such duties. Then, a corresponding fraction of my salary is funded from a different pot of money. At the end of my PhD, this means I will have an equivalent amount of time extra to finish my research before my contract finishes.

I think the system is quite fair, although some tasks — most notably teaching — don't get the actual time assigned, but according to a certain formula. So in practice, I do lose research time by doing teaching, because teaching takes more time than the formula accounts for, certainly if it's a first time. However, it's still much better than a system with no accounting at all, such as I have understood to be common elsewhere in the world.