Just realized, I forgot to caveat this. Any money you want to contribute under the CZTE must go into the traditional TSP, not the Roth. So of that 51k, you can only put 17.5k of it into a Roth, the rest goes into the traditional. So it'll get taxed when you withdraw it, but at least it'll grow tax-free until then.On top of that, if you've already maxed out your TSP contributions ($17.5k), you can still contribute tax-exempt money earned via the Combat Zone Tax Exclusion, in which case it's subject to the annual additions limit ($51k last year, including the $17.5k from non-tax-exempt pay).
https://www.tsp.gov/PDF/bulletins/oc08-13.pdf