Is it OK to reseed a Deterministic Random Bit Generator from itself? I want a deterministic PRNG for simulation purposes, and I don't want to have to worry about spurious correlations like those the DMCT is designed to protect the Mersenne Twister against.  So even though I don't care about cryptographically secure random numbers, a cryptographically secure Deterministic Random Bit Generator seems like the way to go. 

For this purpose, I am thinking of using pycrypto's Crypto.Random.Fortuna.FortunaGenerator.  It only allows $2^{16}$ blocks of pseudorandom data before reseeding because it uses an AES stream in counter mode.  Since I care only about its resemblance to uniform independent draws and not about the entropy which would secure it from cryptographic attack, is there any harm in seeding the generator from its own output?
No, in your situation (where you don't care about security), there isn't any harm in seeding the generator from its own output (or, for that matter, reseeding it by just incrementing the original seed, and resubmitting it).

The chief issue would be if you would fall into a loop (that is, you start repeating outputs); if the seed you generate is 128 bits or more, that is highly improbable for any reasonable number of reseeding.

Now, as you are well aware, a CSRNG is overkill for your purpose; however the question is whether it is cheap overkill.  If the total amount of time your simulation takes is short, or if the bulk of your computation is in things other than generating the random string (and so taking a bit longer in the generation part doesn't significantly slow you down), then it is cheap overkill.