Richard,
It is contrary to the FARs to make modifications to type certificated
aircraft with out approval of the Administator. The age old argument
is "what constitutes a modification".

Most mechanics that work closely with a FAA inspector will tell you
that even the smallest addition of equipment is considered a
modification that requires approval data. Non tso'd varios and gps nav
systems are consider by many to be non-essential equipment and belived
to be leagal to install in type certificated gliders. The only people
that can approve the installation of this equipment in these sailplanes
is the glider manufacturer. Many inspectors and DARs look the
otherway and do not take issue with these instruments.
But the FARs are clear.

When push comes to shove, if you have a standard type certificated
glider, you better have properly approved installations for any
equipment installed. In the case of an accident, the offending
equiptment does not have to be a causal factor of the accident for the
Insurance company to find your glider unairworthy therefore in breech
of the policy.

This seems like a dooms day, naysayer's view but ask your insurance
agent if you are covered when you operate your glider out of annual.
Operating your glider with unapproved equipment installed is the same
thing. This is the main benefit to hold an experimental airworthiness
certificate for your glider.

Rex.