Check for write access instead of HOMEBREW_DEVELOPER.

Library/Homebrew/cask/cmd/automerge.rb

@@ -17,16 +17,17 @@ module Cask
       ].freeze
 
       def run
-        raise "This command may only be run by Homebrew maintainers." unless ENV["HOMEBREW_DEVELOPER"]
+        taps = OFFICIAL_CASK_TAPS.map(&Tap.public_method(:fetch))
+
+        access = taps.all? { |tap| GitHub.write_access?(tap.full_name) }
+        raise "This command may only be run by Homebrew maintainers." unless access
 
         Homebrew.install_gem! "git_diff"
         require "git_diff"
 
         failed = []
 
-        OFFICIAL_CASK_TAPS.each do |tap_name|
-          tap = Tap.fetch(tap_name)
-
+        taps.each do |tap|
           open_pull_requests = GitHub.pull_requests(tap.full_name, state: :open, base: "master")
 
           open_pull_requests.each do |pr|

Library/Homebrew/utils/github.rb

@@ -274,6 +274,19 @@ module GitHub
     search_issues(name, state: "open", repo: "#{tap.user}/homebrew-#{tap.repo}", in: "title")
   end
 
+  def user
+    @user ||= open_api("#{API_URL}/user")
+  end
+
+  def permission(repo, user)
+    open_api("#{API_URL}/repos/#{repo}/collaborators/#{user}/permission")
+  end
+
+  def write_access?(repo, user = nil)
+    user ||= self.user["login"]
+    ["admin", "write"].include?(permission(repo, user)["permission"])
+  end
+
   def pull_requests(repo, base:, state: :open, **_options)
     url = "#{API_URL}/repos/#{repo}/pulls?#{URI.encode_www_form(base: base, state: state)}"
     open_api(url)