Ability to tag a task as a milestone without a milestone path

We would like the ability for project owners to be able to tag a task as a milestone for tasks that may be specific to the project and give them flexibility to define their own milestones based on what the work dictates. This will then enable us to report on the key milestones by project in a meaningful way. For example,

MILESTONE 1 for a project: Launch updated Terms and Conditions

MILESTONE 1 for a different project: Activate Tags via Ensighten