No, I take it back.  I just remembered how the Venetian blinds really work,
and it's not that simple, but it still gives a method that you could use.
Each slat is parented to the bottom bar, and is slaved to the main Pull
dial in reverse.  The bottom bar is also slaved to the Pull dial, but forward.
So when you turn the Pull dial, the bottom bar moves up linearly and each
slat moves down with the same delta, until the slat hits its own Min
value.  After that, it simply moves along with the bottom bar.

In other words, you'd need an intermediate part that moves your real
slave's zTran upward by parenting; and then you'd have the slave responding to the
controller downward, so that its own zTran exactly counters the parented
movement until it bumps into its Min.  After that, the intermediate mover takes the
slave upward with it.  If you don't have a real intermediate that would serve
the purpose, you can introduce a "ghost part" without any geometry.

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