I now have the following working in javascript files to pull in others:

//= require <src>

Next up will be //= require_tree <path>

This also means that I finally figured out my named regexp's from yesterday. It basically boiled down to storing the results in a map. For example:

// Match ^//= require(_tree)? (.*)$
r, err := regexp.Compile(`^//=[[:blank:]]*require(?P<tree>_tree)?[[:blank:]]+(?P<argument>.*)$`)
if err != nil {
    return src, err
}

sm := r.FindStringSubmatch(string(line))
m := make(map[string]string)
if len(sm) > 0 {
    // Take each match and associate it with the named match
    for i, name := range r.SubexpNames() {
        // Don't include non-matches
        if sm[i] != "" {
            m[name] = sm[i]
        }
    }
}

In looking at it just now, I changed the first [[:blank:]] to be optional. Seems like that will give less "WTF?" moments when using it if a space is forgotten.