Hi Thom,
I confirm that Delphix works with entire instance of the DB. So you'll have to work with a new database instance (the virtual copy provisioned by Delphix) that will contain your intended schema(s). What is your concern about working with a new instance?
Storage? The first snapshot of your original DB uses only 30% of the unvirtualized storage space (benefit of the compression services offered by Delphix). Then Delphix will use storage only for recording the changed data: the not modified schemas won't use any further space.
RAM? In the provisioning wizard, you'll be able to set memory parameters to lower size than the original instance (since you won't use all schemas).
Security? You can use an hook script that drop other users before take the new instance available to your db users.
Refresh time? I can assure that the time for refreshing the entire instance will be shorter than a normal exp/imp procedure, if you follow all Delphix requirements.
Keep other schemas unrefreshed while refreshing the intended schema? If you want to take advantage of the powerful features of delphix, in this case, you should use a dedicated instance for your intended schema(s). If you have a cross DB user that join table across separated schema you'll be able to configure an hook script (after refresh event) that creates DBLinks and synonyms.
Feel free to reply any your concern. Me and others from community try to help you, if we are able.
Regards.
Gianpiero