Expanding marginalia header into outer margin without affecting margin text alignment

Hello!

I’m using the marginalia package to format inset margin notes in the outer margins. Doing so shortens the header, which presents two problems for me.

  1. I would like to extend the header into the note margin itself.
  2. I would like the header text to remain centred according to the page text, and so not move when the header is extended.

I’ve attached a couple of screenshots, one of what I have at the moment, and one with what I would ideally like to have. I’m doing this manually but it only works for odd pages. I would like to have it handled on both odd and even pages by marginalia itself. Also when I do it manually the header text moves.

I’ve read through the PDF manual on the GitHub but I’m still unsure of how to achieve this. Does anyone have any ideas how I could do this, or if it’s not possible with marginalia?

Many thanks,
Reiwa