You might be interested in Abiezer Coppe, a Ranter who distributed pamphlets and roamed London "'with his hat cockt, his teeth gnashing, his eyes fixed, charging the great ones to obey his Majesty within him" around the time of Cromwell and England's Civil War - a little late to be medieval, but still.

Some of his pamphlets were in A Collection of Ranter Writings from the 17th Century, ed. Nigel Smith (Junction Books, 1983) reviewed here, but there's more about him... and probably lots of other figures you'd be intrigued by... in The Enduring Problems with Prophecy: from early-Modern times to 2012 and beyond, ed. Dave Evans (Twin Serpents, 2012).

Here's excerpts of Coppe's "A Fiery Flying Roll", printed in 1650.

One good bit:

O London, London. My bowels are rolled together in me for thee, and my compassions within me are kindled for thee. And now I only tell thee that it was not in vain that this form hath been brought so far to thee, to proclaim the day of the Lord throughout thy streets, day and night, for twelve or thirteen days together.

And that I have made such a sign and a wonder before many of thine inhabitants' faces.

Many of them, among other strange exploits, beholding me fall down flat at the feet of cripples, beggars, lazars, kissing their feet, and resigning up my money to them - being several times over-emptied of money that I have not had one penny left, and yet have recruited again ―

…

Wherefore be it known to all tongues, kindred, nations and languages upon Earth, That my most excellent majesty, the King of glory, the eternal God, who dwelleth in the form of the writer of this Roll (among many other strange and great exploits) hath in the open streets, with his hand fiercely stretched out, his hat cocked up, his eyes set as if they would sparkle out, and with a mighty loud voice, charged 100 of coaches, 100 of men and women of the greatest rank, and many notorious, deboist, swearing, roistering, roaring Cavaliers (so called) and other wild sparks of the gentry, and have proclaimed the notable day of the Lord to them. And that through the streets of the great City and in Southwark. Many times great multitudes following him up and down, and this for the space of 12 or 13 days. And yet all this while, not one of them lifting up one finger, not touching one hair of his head or laying one hand on his raiment.

After prophesying a kind of divine revolution, he was tossed in prison, his books were burned by the public hangman, at a pre-trial examination he flung apples and pears around the room... and then near the end of his life, changed his name and practiced medicine.