Dustin,
The Line Repeat can only repeat a static object, and cannot modify this object with criteria or conditions, meaning it's not directly possible to do what you want only with a single object.
One option would be to make a horizontal "line" object (your top border) then snap your line-repeat "box" object (with only the left & right borders), and finally snap another horizontal "line" object to the end.
Note that there seems to be a small 0.01 inch "margin" at the bottom of the line object, so when snapping my repeat-box object to the top line, I had to ajust the vertical offset to -0.01 and that worked...
Also, this will only and specifically work if your data width is static (if only the number of repeats change but the box is always the same width). If it changes, then you're going to need a more complex solution requiring some PressTalk code and a lot more time.
Hope this helps,
Eric.