An API to insert lines into IRC buffers at arbitrary locations would open many possibilities, including but not limited to properly-ordered ZNC playback, even across networks.

Possibly something similar to the following:

weechat_printf_after_date_tags(struct t_gui_buffer *buffer, struct t_gui_line after* , time_t date, const char *tags, const char *message, ...)
weechat_printf_before_date_tags(struct t_gui_buffer *buffer, struct t_gui_line before* , time_t date, const char *tags, const char *message, ...)