- The spec http://www.whatwg.org/specs/web-apps/current-work/multipage/offline.html and other doc always take relative-path examples.
- The same says  "Relative URLs must be given relative to the manifest's own URL"
- Relative path work OOTB with brunch in watch mode as deployed at root or in a subfolder.
- That default value don't affect users who have already upgraded to 1.7.0.

On our side :
- when we cache external assets we use full URIs outside the brunch control scope (that's why I need externalCacheEntries). 
- we build backbone apps with only one entry page and the old-school hash fragments routeur so we don't encounter base-path problems.

The truth is I don't understand your need for a "/static" (appart for some delivery strategy you explicitly set already) and less for a default "/"

As u wish !