I would assume it's pretty straight forward like a snowflake schema, but this is SSAS afterall. I don't work with it that much.

In most of my cases, I try not to let the business units expand on the date dimension to a point of needing this. I do however support multiple fiscal calendars, but that's easily solved if they are all using the same fields, but a key to filter their fiscal based on the business unit in the date dimension itself.

I take it you are trying to support various requirements like Week versus FiscalWeek versus HolidayWeek and so on and so where HolidayWeek may not apply to all units, but maybe just one. Then the question comes to mind, why can't you just create one data mart per unit versus trying to support everything in one?