Alternatives within the data include building what Joe Mako calls a "scaffold" data source that would include all days and then using that as the primary source and blending the secondary to it, or doing the padding in SQL.
When using the Show Missing Values, an alternative to using the WINDOW_AVG function is to use a WINDOW_SUM() and a divisor, such as this:
WINDOW_SUM(SUM([Value]),-2,0)/IF INDEX()<3 THEN INDEX() ELSE 3 END
This calc sums over the 3 values, which will include any padding. The divisor uses the INDEX() function to deal with the first two rows of the partition.
See the attached for an example.