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
At the current time I believe there are no developers with a ArcSDE installation handy, and ESRI is phasing out support for the libraries we used for connecting. MapServer has officially dropped support for ArcSDE, we might have to do the same eventually.
Funding + a test instance might help turning the tables and refresh the existing ArcSDE support, or of course contributions for someone that has development skills and a test instance.
Hum... I still see a ArcSDE doc page in the MapServer docs, yet not long ago there was a discussion about dropping direct support and just relying on OGR on the dev list.
I have an ArcSDE 10.3.1. and 10.4.1 installation and some funding. I have two software developers but they don't have much open source experience. I am interested in working together to resolve this bug.