Gail Stirler wrote an article on Advanced Techniques on Silk Banner Painting in which she includes part of a translation of Cennino d'Andrea Cennini's "Ill Libro dell' Arte" or The Craftsman's Handbook. Cennini was an Italian painter who supposedly lived from about 1370 until about 1440, though this is somewhat questionable. The supposition is that his work was written sometime in the 14th century. The translated section from her article has been copied here verbatim as follows:
X.clxv: If you have to do palls or other jobs on silk, first spread them out on a stretcher as I taught you for the cloth. And, according to what the ground is, take chaboni (vine charcoal), either black or white. Do your drawing, and fix it either with ink or with tempered color; and if the same scene or figure has to be executed on both sides, put the stretcher in the sun with the drawing turned toward the sun, so that it shines through it. Stand on the reverse side. With your tempered color, with your fine minever brush, go over the shadow which you see made by the drawing. If you have to draw at night, take a large lamp on the side toward your design, and a small lamp on the side which you are drawing, this, on the right side; thus there might be a lighted taper on the side which is drawn on, and a candle on the side which you are drawing, if there is no sun. And if you have to draw by day, contrive to have light from two windows on the side with the drawing, and have the light from one little window shine on what you have to draw.
Then size with the usual size wherever you have to paint or gild; and mix a little white of egg with this size, say one white of egg to four goblets of or glasses of size. And when you have got it sized, if you want to lay any diadem or ground in burnished gold, to bring you great honor and reputation, take gesso sottile and a little Armenian bottle, ground very find together, and a little bit of sugar. Then with the usual size and a very little white of egg, mixed with a small amount of white lead, you put on two coats of it thinly wherever you wish to gild. Then apply your bole just as you apply it on panel. Then lay your gold with clear water, mixing with it a little of the tempera for the bole; and burnish it over a good smooth slab, or a good sound, smooth board. And stamp and punch it likewise over this board.
Furthermore, you may paint any subject in the usual way, tempering the colors with yolk of egg, laying the colors in six or eight times, or ten, out of regard for the varnishing; and then you may gild the diadems or grounds with oil mordants; and the embellishments with garlic mordents, varnishing afterward, but preferable with oil mordants. And let this serve for ensigns, banners and all.This indicates to my untrained artistic brain, that the method of silk screen painting was in use during the medieval period. I have yet to find any indication that it was used in the Germanic areas, but at least it gives a lead into the possibility of this technique, as trade between the Italians and the Germans was free and common throughout much of the middle ages.