My suggestion would be a new column in the fact table. My reason behind this is that where I work we don't only use the datamart to feed the cube but to also feed reports directly so I prefer to keep the code logic in the datamart. We use the mentality that the cube is more for self serve and the datamart for the more complex reports. We don't have days here to create reports so when something complex comes up I have about a day to produce the report. I know its not the best practice but neither do I live in a perfect world.

I just finished having a presentation done by a DBA on best practices and from what he showed us all I could think was he has never worked with an MS CRM database.:-)