Value of the Chern-Simons functional for flat connections on $S^3/\Gamma$

For $G=SU(N)$, there is a paper SU(n)–Chern–Simons invariants of Seifert fibered 3–manifolds (Int. J. Math., 09, 295-330 (1998))

For $G = SU(2)$ and the representation given by including $\Gamma$, the value of the Chern-Simons invariant is computed by Millson (Examples of nonvanishing Chern-Simons invariants, J. Differential Geom. Volume 10, Number 4 (1975), 589-600) in topological terms. He explicitly works out the case of lens spaces (cyclic $\Gamma$). It seems that you could get the same method to work for general representations, but it might be fairly involved, because it would depend on a fairly detailed knowledge of the representation theory of your group $\Gamma$.

For $\Gamma$ cyclic, this is probably fairly straightforward, using the fact that any representation splits as a sum of $1$-dimensional representations. There are some additional computations (for $G=SU(2)$ but non-abelian $\Gamma$) in a paper of Tsuboi, On Chern-Simons invariants of spherical space forms, (Japanese journal of mathematics. New series Vol. 10 (1984) No. 1, 9-28).