What ATC facility would be relaying a clearance to an unrecognized control tower? And where are there clearance delivery relays available through UNICOM?
Whichever ATC facility had control jurisdiction over the airspace there. I don't know of any particular UNICOM's that do it. In the age of cell phones and a lot more RCO's there probably aren't very many. Here's the documentation on the subject.
From the AIM
4−1−12. Use of UNICOM for ATC Purposes
UNICOM service may be used for ATC purposes,
only under the following circumstances:
a. Revision to proposed departure time.
b. Takeoff, arrival, or flight plan cancellation
time.
c. ATC clearance, provided arrangements are
made between the ATC facility and the UNICOM
licensee to handle such messages.
4−4−2. Clearance Prefix
A clearance, control information, or a response to a
request for information originated by an ATC facility
and relayed to the pilot through an air−to−ground
communication station will be prefixed by “ATC
clears,” “ATC advises,” or “ATC requests.”
From the Controllers order
4−2−2. CLEARANCE PREFIX
a. Prefix a clearance, information, or a request for
information which will be relayed to an aircraft
through a non−ATC facility by stating “A−T−C
clears,” “A−T−C advises,” or “A−T−C requests.”
b. Flight service stations must prefix a clearance
with the appropriate phrase: “ATC clears,” “ATC
advises,” etc.
From the Facility Operation and Administration order
3−2−5. AERONAUTICAL ADVISORY
STATIONS (UNICOM/MULTICOM)
Pursuant to FCC Rules and Regulations, Part 87,
Subpart C, UNICOM stations are not authorized for
ATC purposes other than the relay of certain ATC
information between the pilot and the controller.
Relay of ATC information is limited to:
a. Takeoff, arrival, or flight plan cancellation
times.
b. ATC clearances, provided there is a LOA
between the licensee of the advisory station and the
FAA facility.