The general-rank specialization formula for refined Poincaré polynomials

Let $N_{r,d}$ be the moduli space of stable rank-$r$ bundles of degree $d$ on a smooth projective curve of genus $g$, and let $\Omega(N_{r,d},q,t)$ denote its refined Poincaré polynomial. The specialization formula.

$$\Omega(N_{r,d}, q, -1)=\prod_{k=2}^r(1-(-q)^k)^{2g-2}.$$

This is proposed as a formula for the $t=-1$ specialization in general rank; the surrounding discussion presents it as conjectural, and no general proof or resolution is supplied.