Beethoven's Immortal Beloved: A first intimation of the text Beethoven really had in mind
Well, yes indeed, as I discovered some years later, Beethoven did have the name of Dorothea in mind for that third movement (AND indeed for every single movement, every single measure of the whole sonata). On this brief fragment of the third movement you can hear it sung that way.
Ibes, Willem (Wim), "Beethoven's Immortal Beloved: A first intimation of the text Beethoven really had in mind" (1998). Music Lectures. 1.