Maggie, a tester on the Rules team, recently wrote a PolicyFromFile sample. The idea here is similar to the PolicyFromService sample that is part of the ExternalRuleSetDemo, except that the PolicyFromFile activity does not use a database and externalizes rules to a file outside the workflow.
At design time, the activity will read from a specified rules file, or if no .rules was defined, a default .rules file with the same name as the workflow file will be created. The name of the .rules file is set on the RulesFileName property.
Get the sample here.