newtone strings just got back to me - they confirm that their custom strings include custom lengths so we can order whatever length is needed. they ask for the length from ball end to nut. btw, i'm also posting this in the sub short thread.
Custom Bass String Configurator – Newtone Strings
I just looked at the web page. Doesn't look like they have flatwound. I did make a few choices to check the price, it showed 450 Euro in the check out, I assume that's for one string.