That statement does not appear in our DDG. Instead,
Equipment not required by the operation being conducted and equipment in excess of FAR requirements are included in the MEL with appropriate conditions and limitations. The MEL must not deviate from the Aircraft Flight Manual Limitations, Emergency Procedures or with Airworthiness Directives. It is important to remember that all equipment related to the airworthiness and the operating regulations of the aircraft not listed on the MMEL must be operative.
There IS a process to defer Nonessential Equipment and Furnishings (NEF) that do not appear in the MEL, CDL, or NEF list:
Controller will use the Maintenance Control Evaluation Process flow chart in conjunction with the Maintenance Controllers NEF Check list (T2091) to determine if the item can be deferred as an NEF item (see “Maintenance Control Evaluation Process” on page 4.00-01-00.4.) If the item meets the criteria to be deferred as an NEF item per the flow chart, then the Maintenance Controller will add the item to Form M2006 (see form sample, “Sample New NEF Item Record Form (M2006)” on page 4.00-01-00.6), and obtain the Maintenance Control Duty Managers approval. The item will then be deferred as an NEF item with a category as applicable.
The end items on the checklist that apply are:
Step 5
Can source (underlying cause) of the discrepancy affect equivalent levels of safety?
UNCERTAIN or NO
Step 6
If applicable, can source of the discrepancy be isolated from other systems with maintenance procedures?
YES or NOT APPLICABLE
Step 7
Defer the item in accordance with the approved NEF program and add item to NEF list
So (at least in FAA land), while a deferral per the AFM might be accomplished as you suggested, it must be done per the approved NEF listing procedure. It is NOT simply up to the mechanic/engineer on the scene.
I realize that procedures may differ in other jurisdictions.