Tux, yes he could put a choke on it but that will be a bit of a shot in the dark. IME, tuning the response of a 2.5 way is not very simple due to measuring at those frequencies along with issues with boundary reinforcement.
Luke, yes, the 2512 is a significantly better driver and I would definitely pay 2x for it. The biggest advantage is matching the SEOS-12's directivity to the 12" woofer vs the 10" woofer. You will get directivity control lower in frequency.
Ultimately if you really want to roll your own with a DSP, you can try any drivers. For value, I would still suggest buying the SEOS/2512 kit as it wold give you something to start from and potentially design your own DSP or passive crossover.