and we know from previous discussions that the current unmerged MTZ format is not the best possible
Perhaps, but it's the only widely supported format that isn't broken by design, so ideally the CCTBX wrapper would provide access to as many features as possible. (I'm assuming that Aimless doesn't read CIF either - correct?)
Graeme: I have no strong opinions on how the API should work as long as existing code does not break, and right now you're the only group trying to output unmerged data. Unless Nick has specific thoughts, I think you should feel free to make it work the way you think is appropriate; we'll probably find things to change later but an imperfect solution is better than none in this case.