How to monkey-patch code that gets auto-loaded in Rails?

I'm monkey-patching a Rails engine with something like:

SomeClass.class_eval do
  # ...
end

The first time I hit the web site, on development mode at least, it works, but the second time it's like my patch never existed. I presume it's Rails auto-reloading the engine (which is installed in vendor/) and not reloading my code. This is Rails 2.3.

Any ideas how to do it so that my code also gets reloaded?