How do I pass interpolation variables to AR validations?

(Rails 2.3.8)

Say I have in config/locales/en.yml:

en:
  activerecord:
    attributes:
      foo:
        bar: 'Long%{br}name'

Now, in HTML I can do:
t(:'activerecord.attributes.foo.bar', :br => '<br/>')

and in LaTeX:
t(:'activerecord.attributes.foo.bar', :br => '\\\\')

However, this is doomed to fail with AR validations, e.g.:

# Model
class Foo < ActiveRecord::Base
  validates_presence_of :bar
end

# View Error
I18n::MissingInterpolationArgument in Foo#create
missing interpolation argument in "Long%{br}name" ({:count=>"1"} given)

So how do I pass :br to 'activerecord.attributes.foo.bar' in the context
of Foo?
Or is my approach wrong altogether?

Thank you

Got this working by using Ruby hashes instead of YAML:

{
  :en => {
    :activerecord => {
      :attributes => {
        :foo => {
          :bar => lambda {|scope, opts| "Long#{opts[:br] || ' '}name" }
        }
      }
    }
  }
}