[PuffinBlue]

Only Apple and Google seem to allow this. I've made the same request to every other email/cal/contact provider I've used but they never do it.

Fastmail seemed interested for about 5 minutes but nothing came of it.

Trying to sync all this custom stuff across walled gardens (iOS and Android bi-directionl sync for instance) isn't the work of just a moment though. So i understand the reluctance.

Perhaps Thunderbird having its own built in flexible fields setup would be possible but you still really need to sync them to be fully useful.