foo.rb

template "/tmp/foo.rb"
  source "foo.rb.erb"
  cookbook "banana"
end

other_shit.rb

include_recipe "whatever_cookbook::foo"

begin
  t = resources(:template => "/tmp/foo.rb")
  t.source "bonkers.rb.erb"
  t.cookbook "definitely_not_banana"
rescue Chef::Exceptions::ResourceNotFound
  Chef::Log.warn "could not find template /tmp/foo.rb to modify"
end