Jono - with an AF lens mounted, if I hit the AF/MF toggle (with it set to MF assist (magnify)), I do get a message saying that this is invalid unless I switch to MF.
However, this isn't really a problem. I still have access to manually focus the lens (I have my camera set to DMF mode - which autofocuses and then allows me to manual focus override simply by turning the focus ring).
Also - as soon as I turn the focus ring on an AF lens, the MF assist kicks in and I get the magnified view.
So this setup still works great, especially for shooters who commonly use MF lenses.