I can’t believe I’ve never tried this before.
Activate Launchbar, type addd (3x d’s) to pull up Add To Day One.
Oddly there are 2 of them for me: one enables only a text string, but the second a modal and temporary window appears that enables you to add content, tag it and send to a specific journal.
This is a write operation so there’s no password prompt to your Day One Journal, and no app to close when you’re done.