Getting some great questions...keep them coming.
Isn't Sola Scriptura anachronistic when placed in a medieval context? The people were illiterate and thus out of necessity they would be dependent on a Magisterium, a clerical ruling class. Whether or not it is the Biblical ideal, is it not unrealistic and historically inaccurate to ascribe this to the pre-Reform bodies?
These are questions both excellent and fair, and often raised both in the pre- and post-Reformation contexts.
It is very striking that among the records we have concerning the three principle groups of proto-Reformers-- the Waldensians, Lollards, and Hussites, there are some key traits they hold in common. They all tended to reject much, most, or virtually all of the ecclesiastical forms of the Roman Church. They rejected in varying degrees, buildings, bells, prayers to saints, prayers for the dead, the priesthood in some cases, and often the worship practices of the Roman communion. Remarks are made not only about church bells and buildings, but against other non-Biblical additions and certainly against man-contrived piety, practices like penance for example.
What was the basis for these arguments, these repudiations? It wasn't sociological, a disenfranchised class rejecting the symbols of the establishment. In every case whether it be the records of an Inquisitor, testimonials, or their own documents, we find an argument rooted in Scripture.
Protestants were from the earliest days, strong proponents of literacy. Though many today would wish they hadn't, Protestants like Calvin and Knox were the earliest advocates of universal education and eventually compulsory education. The original motivation for this was to equip people with the necessary skills to read and study their Bibles. Many readers will know that our modern American school system brought to us by Mann and Dewey was directly inspired by the Prussian model. Prussia was a Protestant Sacralist Utopia, a haven for the persecuted, an enemy of the Catholic Habsburg dominions.
While we can't document the specifics with the medieval groups in the same way that we can with the post-Reformation bodies, we can say with assurance that they elevated the Scriptures and evaluated their contemporary situation in light of them.
They had schools both to teach and study the Scriptures as well as to generate copies of them. Not only in the famed valleys of the Cottian Alps. They had schools throughout northern Italy as well as their numerous cells along the Danube. We can safely assume the thousands who were located north of the Danube in South Bohemia echoed this practice. There are also numerous accounts of the Waldensians having memorized large portions and in some cases the entire New Testament.
So while many were undoubtedly illiterate, the proto-protestant groups placed a great emphasis on literacy in order to read the Scriptures, and for those who couldn't read, they could listen as large portions of the Bible were recited. In other words the Bible was the centerpiece of their spiritual identity.
They obviously exhibited a passion and love for the Scriptures which we lack today. The Scriptures stirred within them a zealous courage willing to suffer persecution and the flames of death.
What of extra-Scriptural revelation? Charismatic gifts, prophecy etc…?
I have not found any record of any of these groups embracing such ideas. Charismatic theology is actually very much in accord with Roman theology and this is equally true today. Charismatic Christianity is now the largest grouping in the world and outside of the West, it is also quite dominant in Roman Catholicism. The Magisterium and ultimately the Papacy were and are viewed as wielding a Prophetic office and certainly medieval piety allowed for ecstatic visions etc….
That said, I will emphasize once again that just as today, there were a multitude of different groups spread across Europe and it is likely there were some who were led by self-proclaimed prophets.
There were manifestations of Charismatic type activity in the post-Reformation period. Sadly the record of Comenius (1592-1670) of the Bohemian Brethren is stained by his adherence to Nicholas Drabik whom he believed to possess prophetic powers. Though Comenius was wrong in trusting his friend, his work is still to be esteemed.
Another rather startling episode involved the Camisard War in the Cevennes. This took place after Louis XIV revoked the Edict of Nantes in 1685. By the turn of the century, the Huguenots or French Calvinists had been driven to desperation. Their world had been turned upside down and destroyed. Many had fled, but there had also been widespread apostasies, Reformed Protestants returning to the Roman fold, and the faithful were living a desperate existence in the wilds and mountains. The White Shirts, or Camisards began to listen to certain 'prophets' and eventually turned to violence, a long and painful guerrilla war against the French crown. There are interesting connections between some of these prophets, their exile, and the later rise of the group that came to be known as The Shakers.
And finally there are certainly evidences of some of the Anabaptists engaging this type of Charismatic activity.
The issue of Canon is critical even for today and is concept that few Evangelicals seem to grasp. They pay a certain level of lip-service to the idea of the Bible Alone, but then exhibit a total misunderstanding of what that means or in many cases their practice demonstrates a flat rejection of the principal.
I am not sure to what degree the Waldensians and others worked out the theological basis for Canon, but in practice if not in theory they held to it with a vibrant tenacity.