Freqphase is just corrections via tube length, at least that's my understanding. If a resheller measures carefully, they should be able to maintain it.
I've read Phil with Cosmic Ears is confident in reshelling them. And I wonder if Peter of Custom Art could reshell them in silicone. That would be awesome.
That is my understanding as well - tube length, tube diameter and filter placement. If you look at JH 13/16 from before the FreqPhase implementation, you will see the High Frequency BA mounted extremely close to the canal opening - almost at the end - with FP, the HF BA is much further back in the assembly. (My V8 has almost the same location - further back in the headphone)