Can an Encoding Altimeter be installed as the primary Altimeter (Pilot's) if the Encoder portion of it has been disabled or has not been wired to the Transponder?
ANSWER: No avionics component can be installed in a U.S. registered aircraft unless that component was produced under a production approval, and the data used to install and/or alter that components installation is FAA approved. Having said that, I refer you to - FAR 91.215 ATC transponder and altitude reporting equipment and use - for the operational requirements without an encoding altimeter.
The person who asked this question did further research and found a document published by the FAA that addresses this subject in detail. The document is called a Flight Standards Information Bulletin for Airworthiness. You can download a copy by clicking this link FSAW-97-02
I would like to thank her for sharing this information, and I'm sure that other users of this site will find it quite useful in addressing this subject.