I like the ideas given thus far, particularly adding a tube under the shield and/or breast compression to keep the flow up.  I would caution that these ideas may not allow the baby to stop having a bottle cold-turkey, so if a bottle will still be part of the equation, I'd help mom choose a shape that allows the baby a latch similar to her latch at the breast, and maybe see if the flow of the bottle she is using could be reduced.
On another note, do you think it's perhaps a stamina issue?  That will take a bit more work.  In my state, the Infant-Toddler program follows all preemies with an OT or PT.  Does the mom you're working with have this support as well, or could she request it?  (it's free)
Amy Peterson, IBCLC
www.breastandbottlefeeding.com has some good pics of mouth placement on bottle

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