How is a 14 CFR 101 sUAS NOTAM filed for an unregistered aircraft?
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)