Jekyll maps files like _posts/YYYY-mm-DD-the-title.md to URL paths like /YYYY/mm/DD/the-title.html, but those paths can also be customized. My blog, for example, prefaces the category before the year. I'm sure other SSGs support this as well.
I appreciate some of the frontmatter is needed for publish to function, but you can't conceivably process other SSGs' path mapping. So what about trying to match heuristics?
If you have the source and destination paths for posts e.g., _posts/ vs. _site/, perhaps it's common enough that you can look for "the-title" in file names, stripping off the year like someone did recently for Jekyll.
Alternatively, may sequoia should support a hook that every blog could add to handle their own mapping.
Another idea might be to allow some lightweight templating. With the date parsing for Jekyll, and turning all frontmatter fields into template fields, we might be able to supply a template in sequoia.json e.g., I have a category frontmatter field, so if sequoia picked out the date components, I could do something like:
"{category}/{year}/{month}/{day}/{title}.html"
Though, in this case, the {title} here is the slug - or at least just the part that represents the title.