So I finally got back to this. My site crashes when I point at the prepared D7 database (modules disabled, etc.). I tracked it down to a database query in path.inc. Basically it is querying the url_alias table to get the source for home, and expecting one column to be langcode, but in D7 it is language, not langcode, to the query has a fatal error. This is happening during bootstrap so the site never comes up for me to run upgrade.php, which is where I assume such things in the database would be converted. I don't know if drupal used to use langcode and switched column names and backdrop hasn't taken that into account yet, or what is going here.
SELECT source FROM url_alias WHERE alias = 'home' AND
langcode IN ('en', 'und')
ORDER BY langcode ASC, pid DESC
(I've replaced the tokens)