SSAS - Creating and Using a Writeback Measure Group

Imagine you have spent the last 9 months developing a sales Data Warehouse and an Analysis Services cube. Your end users love what you have built for them so far but it's never good enough right? You have a new requirement to allow your users to enter in projected or target sales for future months. You think about your options for a while and narrow it down to 3 possible ways to solve the problem.