What if the users changes it's workschedule? You'd have to perform an administrative task for each user that decides to change the days they're on the job. Giving the user the ability to provision it's own variables based on application objects (user option - resulting in an environment variable) would be the path of least resistence. Also, workspace containers implies using specific servers/agents combined with access control rules..
I think there's not really a very easy way to do this, but I think I'd use Workspace Containers for this.
Get a list of which people work parttime and on which days, then create a Workspace Container for each possible combination. (For example, a Workspace Container "Mondays, Tuesdays and Fridays", a Workspace Container "Mondays, Tuesdays, Wednesdays and Fridays", etc) Give each parttime worker the correct Workspace Container.
Then you can set up a global environment variable that gets the correct text as value depending on the user's Workspace Container.