Jason,  I've had my hands on only two Duo-Art stacks, but they both
have a long bar; it's not primaries but bleeds.  I believe that 'tis
standard.  It's a long row with input and output tubes, then there is
a cloth covering a long trough with rows of pin-holes opening to all
of the notes.  I haven't seen the action of a Weber Pianola grand piano
so I can only comment on what may be what you've seen.

To convert it to Duo-Art is not impossible, but you will need to
arrange for the following parts:

Duo-Art tracker bar: best for you to have a pre 1923 one as it has
  88 notes.
Duo-Art expression mechanism and valves
Electric motor and pump
Modulator mechanism
Cut-out for Duo-Art for 88-note play (and the top and bottom four notes
  if you're using 88-notes when Duo-Art play)

You will need then to split the stack, adding a vacuum input on both
sides (not just one).

It's probably easier and more economical to get an original Duo-Art
than to convert yours, unless you have a phenomenal piano.

Eliyahu Shahar