In november last year Hans Hagen was so kind to send me a copy of
back-u3d.mkiv which is
now included in the latest beta. As Hans mentioned the code he gave me is
untested.

After a couple of tests here are my findings

*) The preview does not work at this moment. I only see a light blue square
which can be activated.
After which the 3d viewer shows the embedded 3d image
*) Specifying several views as shown in back-u3d.mkiv shows up in the
resulting pdf as "unknown view"
*) I also experimented with different values for coo, c2c, roll etc. When
not specifying any of these settings
the embedded 3d is always shown with the incorrect roll i.e the roll is
always 90 degrees off

For anyone interested, the 3d file was produced with Asymptote 1.91 (
http://asymptote.sourceforge.net/)

=============== Context sample code ==============================
\starttext
\startluaparameterset [u3d:myset:display:1]
    toolbar=false,
    preview="prcu3d/loading.png",
    view = {
        name = 'default',
        bg = {1,1,1},
        mag = 25,
        coo = {2,2,2},
        c2c = {0,0,1},
        rot = {40,0,60},
        roo = 2,
        lights = 'CAD'
    }
\stopluaparameterset
\useexternalfigure  [cubes]  [prcu3d/cubes.prc]  [width=\textwidth,
height=0.5\textwidth, display=u3d:myset:display:1]
\placefigure[here]{3d cube}{\externalfigure[cubes]}
\stoptext
========================== end of Context sample code
=======================================