The Xivkyn motif chapters will only drop from special treasure chests within the Imperial City. Saving your crafting writs will do nothing to help you obtain these.

Glass motif chapters are obtained as rewards from crafting writ boxes (only clothing, blacksmithing, and woodworking), so theoretically saving your writs until the IC patch launches would be beneficial. However, if the boxes are coded in such a way that the game knows if they were obtained pre vs. post IC, then the pre-IC boxes might not contain glass chapters. I know people have asked for clarification on this before, but I don't remember a dev ever responding. I would say that if you are okay with holding onto your writ boxes until IC (i.e. have inventory space and/or don't desperately need anything from them), it couldn't hurt anything and might put you ahead of the curve when it comes to earning the glass motif.