I was doing some thermodynamic calculations and I found the need for a multivariable analogue of the triple product rule. Basically I had a set of $m$ functions $z_i(y, x_1, x_2, \ldots, x_m)$, and I needed to calculate partial derivatives of changes in $x$ resulting from a change in $y$ where all $z_i$ values are simultaneously held constant, i.e., I want to calculate:
$$ \left(\frac{\partial x_1}{\partial y}\right)_{z_1, z_2, \ldots, z_m}, \ldots, \left(\frac{\partial x_m}{\partial y}\right)_{z_1, z_2, \ldots, z_m} . $$
As a reminder, the regular triple product rule would tell us the following, which unfortunately is not helpful in the multivariable case:
$$ \left(\frac{\partial x}{\partial y}\right)_{z} = - \frac{\left(\frac{\partial z}{\partial y}\right)_{x}}{\left(\frac{\partial z}{\partial x}\right)_{y}} \qquad \qquad \text{when $m=1$}. $$
To compactify, let's use linear algebra notation and write our problem as us knowing $\mathbf z (y, \mathbf x)$, and wanting to find:
$$ \left(\frac{\partial \mathbf x}{\partial y}\right)_{\mathbf z} = ~ ??? $$
I believe I have the answer to this (which I will post as an Answer), but I am unsure as I cannot find it in standard places. This raises a follow-up question for me, which is whether this has a name?
Example
I came across the need for the above rule when trying to work out heat capacity in the grand canonical ensemble, in the case of a multi-component system (multiple chemical potentials $\mu_1, \ldots, \mu_m$ and multiple particle counts $N_1, \ldots, N_m$). In this ensemble our natural variables are temperature $T$ and $\boldsymbol \mu$, but heat capacity is a constant-$\mathbf N$ derivative. We have:
\begin{align} C_V & = T \left(\frac{\partial S}{\partial T}\right)_{V, \mathbf N} \\ & = T \left(\frac{\partial S}{\partial T}\right)_{V, \boldsymbol \mu} + T { \left(\frac{\partial S}{\partial \boldsymbol \mu}\right)_{V, T} }^T \left(\frac{\partial \boldsymbol \mu}{\partial T}\right)_{V, \mathbf N} \qquad \text {(using regular multidimensional chain rule)} \end{align}
So the question is then how to get $ \left(\frac{\partial \boldsymbol \mu}{\partial T}\right)_{V, \mathbf N} $, i.e., here $\mathbf x = \boldsymbol \mu$ and $\mathbf z = \mathbf N $. Besides grand canonical ensemble, similar examples would appear if we are considering ensembles involving vectorial quantities, such as magnetization or angular momentum.