Is it possible to limit which users are available for selection in my user enumerated list (i.e. Responsibility) for certain workflow steps?
Yes in the Cross-Platform Client (2008 R2) the users available for selection in enumerated fields are configurable using the workflow designer. To use the workflow designer an Enterprise Advantage license is required. You can download StarTeam 2008 R2 from here: http://www.borland.com/products/downloads/download_starteam.html