Hey Kurt,
You can accomplish this using Security Roles within OnTime. The SCM tab will only be shown for items in which the user has access to work with the SCM tab for the project in which that item lives. The specific priviledge you're looking for is called "Defect SCM" (or the appropriate item name). For example, you could create security role that only grants access to the SCM in certian projects then grant access to that role in addition to your other roles.