The decoration feature of PICO-8 v0.2.2 could also be useful to also draw dakuten / handakuten marks above the kana characters, rather than to the side of them, but sadly, since the current syntax only allows them to be shifted horizontally between -2 and 1 pixels.
Maybe it's worth it to adjust the "\v" character syntax so that it works differently after wide characters, or add a different one where the horizontal shifts use 3 bits instead of 2 (and allow for horizontal adjustments between -4 and 3 pixels?).