After a couple of days with a 18350 in, I am coming to the same conclusion as you, however I did find some 18350s the other day that were 1200mah. Now if only I can find those again, I really like the size of it with the 18350.
Why wouldnt you have been able to, they would have only had to mill hole and BAM you still have ego threads, if the SPHINX mod can why wouldnt these ones?
I have not seen any 1200ma 18350s that were not protected =( If you find them please send me a link.
Also still looking for ANY sort of 18500 tube thats not 50+ dollars to fit my evic!!