Selecting a large NUMS Safe prime
Well, the obvious answer to 'how to find a Safe prime that is also a Nothing-Up-My-Sleeve' number would be to take one of the primes listed in RFC 3526.  These primes (which come in several sizes) are all of the form $p = 2^n - 2^{n-64} - 1 + 2^{64} \cdot ( \lfloor 2^{n-130} \pi \rfloor + i )$, where $i$ is the smallest nonnegative integer such that both $p$ and $(p-1)/2$ are prime.  Having most of the bits within the modulus be from the binary expansion of $\pi$ would appear to satisfy the Nothing-Up-My-Sleeve property.

Also, I don't know if it is wise to pick a generator; after all, from the hash $a^m \bmod p$, the attacker can deduce the value $m \bmod 2$ if $a$ is a generator.  It would seem more prudent to have $a$ be a quadratic residue (and with the RFC 3526, along with any safe prime that is $p \bmod 8 = 7$), $a=2$ is such a quadratic residue, as well as making the exponentiation operation slightly cheaper.