Given that the dummy app is not supposed to be tied to the parent gem except for some vague names, that may do it:
cd some_path_where_your_engine_IS_NOT
rails plugin new YOUR_ENGINE_NAME --mountable --dummy-path=spec/dummy --skip-test-unit
mv YOUR_ENGINE_NAME/spec/dummy /real/path/to/YOUR_ENGINE_NAME/spec
rm -rf YOUR_ENGINE_NAME # cleanup useless cruft
Also, you may be interested in this answer to generate the app with the exact same rails version.