I know this hurts, but you should have a talk with the people developing your cube about adding YTD, MTD calculation to the cube.
Some stuff can be done in Tableau with writing MDX in the calculated members dialog - but you need to know your MDX and have a dummy dimension set up (you need the cube developer for that as well)
Hey Greg - I had typed out an answer and then the website decided to eat it. I only managed to get a quick screenshot when I realized what was happening. It's below - sorry about the format.
I'm just about to head out until Monday - if you can't read this I can email you the larger copy then. The obscured first sentences are: "You can use tuples to get a measure value for a given member. If you haven't looked at the AdventureWorks sample .twbx, take a look."