teropa - it strikes me that every time a professional pilot says it can't be done you don't like it. From the tone of your replies I can only presume that you've already made your mind up.
Your technical knowledge is impressive for someone who isn't even type-rated but it is far from complete. Many older 757's and 767's have had retrofits to many of their sysyems, not least the aotomatic arming of the autoplilots.
You state that you would use "vertical speed" mode. In my opinion this would be the least desirable because it offers NO speed protection.
I could create a massive list of questions that you probably haven't considered but I don't want to use up all of Danny's bandwidth.
Anyway, if you're actually interested in my two cents worth - and I speak from considerable experience (several thousand hours on 767's and a similar number on 757's) - then yes I suppose it may be achievable but in all probability highly unlikely.