明示した変数だけ環境変数で上書きできる感じのやつ

override.cap

# vim: ft=ruby

namespace 'load' do
  task :defaults do
    set :overridable_attributes, :branch
  end

  task :override do
    attrs = Array(fetch(:overridable_attributes)).compact
    attrs.each do |attr|
      name = attr.to_s.upcase
      set attr, ENV[name] if ENV[name]
    end
  end
end

Capistrano::DSL.stages.each do |stage|
  after stage, 'load:override'
end