That is a question that has been posed to me a number of times. So, let's look at it both from the perspective of the past and how it fits within the Constitution.
I have found reference to "extra-legal" in certain writings about Committees of Safety. Extra-legal means outside of the protection of the law. It does not mean illegal. I have found nothing that indicates that they were deemed illegal by the Crown, though once they became active, their actions, in many instances, were considered to be illegal. I have found nothing where any effort was made to "arrest" any Committees of Safety, though Sam Adams and John Hancock were surely targets of such effort on April 19, 1775.
Now, we shall visit the Constitution - specifically, the First Amendment:
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
A Committee of Safety, if its concern is to establish an infrastructure, to be implemented in the event of a failure of the existing government, so that "Civil government" can be maintained, as was only existent in our Revolutionary period, then it is nothing more than Freedom of Speech in a body Peacefully Assembled. Like the spare tire in your car, there is always the hope that it will never be needed. There is little doubt that the same is true of this infrastructure that the infrastructure created by the Committee of Safety is, likewise, something that we hope will never have to be utilized. To meet and discuss and plan for something as important as the protection our lives, families, and property, by planning for the maintenance of civil government, cannot be illegal, by any stretch of the imagination.
By being outside of the protection of the law simply means that it is not protected, specifically, however, at the same time, it does not fall outside of the retained rights addressed in the Ninth and Tenth Amendments to the Constitution, to wit: