FixtureReplacement overriding How to

Is anyone familiar with overriding the values for
FixtureReplacement.defaults_file

I've reviewed the rspec testcases. Changing the defaults_file on
first initial request fails; as it tries to load the default file
first "#{rails_root}/db/example_data.rb", the remaining request will
initialize the defaults_file correctly with no errors.

The easy approach would be to modify the file which contains the
defaults_file setting in FixtureReplacement.

Is this the rails way/best practices for overriding plugins.

Any ideas?

Thanks in advance

Reggie