After following the instructions detailed in the GeoServer documentation (http://docs.geoserver.org/2.7.1/user/data/database/arcsde.html), a direct connect does not seem possible. The same issue has been around with earlier versions as far back as 2.3.2. ESRI no longer supports three tier connections and insists on using direct connections from version 10.2 onwards (http://blogs.esri.com/esri/supportcenter/2015/02/11/where-is-the-install-for-arcsde-10-3/). To continue using GeoServer, a direct connection is vital if using an ArcSDE vs 10.3 environment.
An example of the error returned is as follows:
2015-08-31 14:58:33,365 WARN [data.store] - Error obtaining new data store
org.geotools.arcsde.ArcSdeException: Can't create connection to none for Session #8[SDE error -66][Error desc=SPECIFIED PARAMETER VALUE IS INVALID.][Extended desc=
32-bit Oracle Client installed
GeoServer 2.7.2 + ArcSDE extension
ArcSDE version 10.2 over an Oracle 11g database