How is a 14 CFR 101 sUAS NOTAM filed for an unregistered aircraft? FAA promotes the utilization of NOTAMs to announce sUAS operations, and requires it in 333 exemptions and some 107 waivers. However, sUAS (drones / model aircraft) under 14 CFR 101 have been determined to not require registration.

Drone NOTAM entries are not possible without a registration number.

How, without registering a 101 model aircraft, may one file a Drone NOTAM?

EDIT 15 June 2020:  Regulations have changed, and now Exception for Recreational Flyers requires that aircraft 250g or greater be registerd.  This addresses this question, as the question was relevant to former part 101 recreational flyers, which has been changed to now require registration.

However, aircraft below 250g do not require registration, and a pseudo identifier, created by the owner may be used as a registration identifier.

I will create an answer for this question reflecting the current regulatory environment.
All part 107 aircraft must be registered, regardless of weight.  The question addresses the case of Part 101 flyers, who were exempt from registration.

Regulations have changed, and the Exception for Recreational Flyers now requires aircraft above 250g, used for recreational flights, to be registered.  Hence a registration number would be available for drone NOTAM filing.

In the case of aircraft below 250g, which are not required to be registered under the Exception for Recreational Flyers, an identifier may be offered by the filer of the NOTAM.  (eg: MONGO2025551212, which might be a name and phone number)