Can I access the .blend file path during save_pre on the first save?

Can I access the .blend file path during the first execution of

Here’s some context:
My addon allows users to import several files, but the data would be referenced rather than saved as part of the .blend file. So I need to store the path to the imported file. I would like to be able to save the path as a relative path. Only problem is, before the .blend file is saved, there’s nothing to make the path relative to.

I was thinking I could capture the save_pre() event, grab the intended .blend file path, go convert the absolute paths to relative ones, then let Blender save the file. However, during my testing, I found that is not set until after the save_pre() event. It’s available in the save_post() event, but by that point it’s too late to make the change; the user would have to save the file twice the first time.

Any ideas? Is there some equivalent of