Foreign pilots flying as PIC in a US "N" numbered glider in the USA
should consider the requirements:

CFR 61.3(a) states that a foreign pilot flying a civil aircraft of the
United States must have some level of FAA Pilot Certificate.

Option 1. You could earn a "real" US Pilot Certificate by taking
ground and flight training and take the written knowledge test and a
flight test for Private Glider.

Option 2. You could apply to the FAA for a "Restricted" Private pilot
certificate.with glider category rating.

The process for the "Restricted" certificate “based on the foreign
license” has changed and requires a "Verification of Foreign License"
by the FAA. Application should be made to the FAA at least three
months before flying in the USA.
FAA webpage: http://www.faa.gov/licenses_certific..._verification/

The “Special Purpose Authorization” of CFR 61.77 is meant to apply to
commercial aircraft operations.

TSA (Homeland Security) issued a waiver that excludes glider pilots
from background checks and fingerprinting before training for the US
glider pilot certificate.

A Flight Review may be required under CFR 61.56 and 61.2(b).

CFR 91.307 will apply to your 180 day parachute repack used in a US
aircraft.

There was some speculation for the 2012 Worlds at Uvalde that a waiver
from the FAA might be requested by the organizers. I do not know if
this was approved.

Note that the owner of the US glider should check with their insurance
broker to assure coverage is in effect for loaning or leasing of the
glider to properly certificated foreign pilots.

The owner should prepare the glider for a FAA “Ramp Check” items
including aircraft airworthiness and registration certificates, weight
and balance, flight manual (if required by the TCDS), placards,
airspeed markings, seat belt tags, etc.
Sometimes the FAA wants to review the aircraft logbook for the annual
inspection and compliance with all Airworthiness Directives,
especially the repetitive AD’s. This request is supposed to be in
writing but I understand that logbooks were requested for the
towplanes at a recent glider event.

Best to have all of your pilot and aircraft paperwork available and up
to date.

I’m not happy about all of these regulations so please don't shoot the
messenger.