How do I pass interpolation variables to AR validations?

(Rails 2.3.8)

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

        bar: 'Long%{br}name'

Now, in HTML I can do:
t(:'', :br => '<br/>')

and in LaTeX:
t(:'', :br => '\\\\')

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

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

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

So how do I pass :br to '' 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" }