No unfortunately it is not possible to lock a property.
It may be a good idea to make a wish with a poll for this (I don't remember seeing one).
In the schedule criteria you can search for elements with properties that 'has default value' or 'has custom values' or 'is undefined'.
You can also set up a Graphic Override rule for this so you can highlight them on plan rather than using the schedule.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11