Currently, my company utilizes agile as its development principal. I was approached by my boss to determine some methodology for determining the amount of work a project manger does on a given project in flight. To be honest, I can't really think of anything fool proof.
I guess the best question is how do we assess how busy, on a day to day basis, a project manager is?
Remember that ANY metrics you can come up with is most likely going to be gamed.
[ Do I get a badge for on-topic link to Joel On Software? :) ]
Having said that, you can try a union of the following approaches:
Developer feedback!!! (e.g. a good PM's feedback would be "I had problems X, Y and Z and he made them disappear"). Not so good for measuring how "busy" a PM is but really good for measuring how effective he/she is.
Volume and rated clarity of project plans (easily gamed)
Rate of change of project plans (easily gamed)
Amount of meetings/meeting time (easily gamed)
Success rates of projects (on timeliness vs. % of features delivered vs. customer satisfaction). Not easily gamed but devil's own work to normalize this across projects.