reference a project in a different workspace?

I'm not quite sure what you mean by "reference." If it's just a matter of coding against the classes you've developed in another workspace, you certainly can--even if you don't want to deploy the project as a library. Just set your current project's "additional classpath" to include the "classes" directory of the project you want to use.

Even if, say, you want to use business components from the other project, you can "import" the other project's business components (select the business component package XML file to import them).

If you want to do something fancy, like actually ensure the other project gets rebuilt every time one of your projects gets rebuilt, then I think the only way to go is to add the project to the current workspace. (A project can be in more than one workspace; just "add to workspace" and browse to the other project's jpr file.) I'm not terribly sure I recommend this except in extremis, though--if a project is open in two workspaces, dealing with, e.g., whose job it is to actually change files in it can be difficult.