I raised this question at https://wiki.openstreetmap.org/wiki/Talk:Key:addr:flats#Exceeding_255_characters previously. Is there anything you can point me to that indicates the :2 :3 etc. key suffix is in use already for overflowing tag values?

On another note, thinking about this more, perhaps there is a different syntax we can come up with that represents these long addr:flats values in less characters?