Symptoms

I'd like to have my workflow assign destination users based on the environment. Is there a way to do this that is process specific, rather than editing the Global Environment Library?
 

Diagnoses

Seems like a process specific set of users is needed. A K2 role can be made and applied for one specific process, this will most likely be the best route for gaining the functionality desired.
 

Resolution

You are able to configure a Role which contains the destination users of your environments. So for example you could have a role set for a certain group in your Dev environment and another role that contains the users to be used in the Prod environment. You can also have this specifically built for a certain process through the K2 Workspace or through the Process Management -> Roles in Sharepoint. You can create a role in the Administration page of Sharepoint 2010. If you click on roles and then the action button there will be a "New Role" button to start creating a new role. Here you can add in your users that you want by searching for them after clicking on the actions button at the bottom of this form, click add role item. After creating the roles you can then add them to a process through workspace or Sharepoint, whichever you prefer. The path to add roles in workspace is Mangement console -> Workflow Server -> Processes -> Drill down into the process you want -> roles.

After the roles are added to the process you will be able to see them in the context browser when choosing the destination users, just make sure you are looking for roles, not users or groups.

There are a few pieces of documentation on roles that I believe will be quite useful for you, see them below:

http://help.k2.com/onlinehelp/K2blackpearl/UserGuide/current/webframe.html_Reference-WS_MCon-Workflow-Processes_Roles.html
http://help.k2.com/onlinehelp/K2blackpearl/UserGuide/current/webframe.html_PP-Process_Management_-_Roles.html
http://help.k2.com/onlinehelp/K2blackpearl/UserGuide/current/webframe.html_Reference_-_K2_Roles.html