What's the partition coefficient (D) of Nb for apatite?
Yes, in the paper Trace element partitioning between apatite and silicate melts (Prowatke and Klemme, 2006), where they present experimental apatite/melt trace element partition coefficients for a large number of trace elements, including $\ce{Nb}$.  The experiments were performed at pressures of 1 GPa and at a temperature of 1250 C.

What the experiments revealed were:


An average of $D$ = 0.0018 for $\ce{Nb}$ (this value is an average of Table 5: Trace element partition coefficients between apatite and different silicate melt of the linked document).
$D_{Nb} \approx D_{Ta}$
The partition coefficient was not significantly affected by changing melt compositions.