nt.number theory - What's the status of the following relationship between Ramanujan's $tau$ function and the simple Lie algebras?

Qiaochu asked this in the comments to this question. Since this is really his question, not mine, I will make this one Community Wiki. In MR0522147, Dyson mentions the generating function $tau(n)$ given by
$$ sum_{n=1}^infty tau(n),x^n = xprod_{m=1}^infty (1 - x^m)^{24} = eta(x)^{24}, $$
which is apparently of interest to the number theorists ($eta$ is Dedekind's function). He mentions the following formula for $tau$:
$$tau(n) = frac{1}{1!,2!,3!,4!} sum prod_{1 leq i < j leq 5} (x_i - x_j)$$
where the sum ranges over $5$-tuples $(x_1,dots,x_5)$ of integers satisfying $x_i equiv i mod 5$, $sum x_i = 0$, and $sum x_i^2 = 10n$. Apparently, the $5$ and $10$ are because this formula comes from some identity of $eta(x)^{10}$. Dyson mentions that there are similar formulas coming from identities with $eta(x)^d$ when $d$ is on the list $d = 3, 8, 10, 14,15, 21, 24, 26, 28, 35, 36, dots$. The list is exactly the dimensions of the simple Lie algebras, except for the number $26$, which doesn't have a good explanation. The explanation of the others is in I. G. Macdonald, Affine root systems and Dedekind's $eta$-function, Invent. Math. 15 (1972), 91--143, MR0357528, and the reviewer at MathSciNet also mentions that the explanation for $d=26$ is lacking.

So: in the last almost-40 years, has the $d=26$ case explained?