Separate sleeves pinned to the dress were a rather common feature in late medieval fashion. Examples are found in the artwork of various artists from different regions in Europe. The following pictures which show pin-on sleeves are by different german artists and Rogier van der Weyden, a flemish artist that I like a lot because of his love for detail.
Apparently, the sleeves could vary in length: The pin-on sleeves of the wise virgin in the first picture are rather short when compared
to the other pictures - they only reach to the middle of the upper arm. The sleeves of the violent wife in the second picture end below
shoulder level, whereas the sleeve of the spinner (picture 3) almost reaches shoulder level.
In the fourth picture you can see why I think that pin-on sleeves were (and are) quite handy: You can easily remove it and roll up the sleeves of your undergarment if you have work to do like the women testing the temperature of the bath's water has just done. Standing right behind her (see enlargement) is another women who wears green sleeves pinned to her gown which again do not quite reach her shoulder.
The second row is a small collection of pictures from the flemish painter Rogier van der Weyden. All these sleeves seem to be made of expensive cloth: brocade (picture 5), damask (picture 6 & 8) or silk (picture 7). Two of the sleeves reach up to shoulder level (picture 5 & 6), the other two are shorter (picture 7 & 8).
In picture 5 you can see the white undergarment sticking out next to the sleeve indicating that the sleeves of the blue dress are very short. In picture 7 (with shorter pin-on sleeves) the sleeves of the red dress are slightly longer, so possibly the length of pinned sleeves depended on the sleeve length of the dress worn below. In picture 5 you can clearly see the pin that keeps the expensive sleeve in place (This is what I mean with 'detail' in Rogier van der Weyden's paintings!). The sleeves in picture 7 show nicely the position of the seam on fitted medieval sleeves: on the back of the arm instead of the underside as in modern sleeves. The sleeves in the last picture are different from the others - they are not fitted but slightly puffy yet gathered again at the wrist.
April 02, 2010: I've made a short pair of pin-on sleeves for my short sleeved working dress. I've used reddish, medium weight woolen fabric and lined the sleeves with white linen.