So I think this is possible. I've worked up a quick example, so let me know if/how it differs from your situation...
So we can first (as it looks like you've done) show the "missing months" using the Show Missing Values feature (which works for Dates and Bins)
Now in order to "access" these "densified" marks we can use the Previous Value function....
[Sales - Dense]
IF ISNULL(SUM([Sales])) THEN PREVIOUS_VALUE(SUM([Sales])) ELSE SUM([Sales]) END
Hopefully that does the trick, but let me know if not (and where it fails), and I'm sure we can refine the logic.
This issue kept me using SQL calculation for YTD. And you solved it.