One thought that has occurred to me is that Matthew's gospel gives a further indication that the "missing" ending may never have existed or at least was lost extremely early. We all know (I think) that Matthew borrows heavily from Mark. In fact Mark is Matthew's primary source for narrative details. When we read the end of the Gospel According to Matthew, of course we get the cute story about the guards which "is told among the Jews to this day" (Kudos to Matthew for this nice bit of apologetics). This is followed by chapter 28, verses 16-20
16Now the eleven disciples went to Galilee, to the mountain to which Jesus had directed them. 17When they saw him, they worshipped him; but some doubted. 18And Jesus came and said to them, ‘All authority in heaven and on earth has been given to me. 19Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20and teaching them to obey everything that I have commanded you. And remember, I am with you always, to the end of the age.’This brief and simple appearance story pales next to Luke's and John's memorable versions. However, if we line it up with Mark's passion narrative a curious thing happens. Matthew's appearance story is precisely what we would expect to follow Mark 16:8, namely the disciples meet Jesus in Galilee just as the young man in the white robe instructed the women at the tomb saying, "he is going ahead of you to Galilee; there you will see him, just as he told you." Mark 16:7.
Is Matthew stuck with a copy of Mark that ends without the promised reunion? Does he feel the need add exactly the scene that so many scribes felt compelled to supply as well? I must admit that I never felt completely satisfied with the conclusion that Mark ended so abruptly but this little detail has me rethinking my position.