ParallelPeriod returning null for Feb. 29 in date dimension
Alright, I think I've got a workaround I can live with. First, I created a named set in the cube, called Feb29:

Filter(
    [Post Date].[Post Date].[Day].ALLMEMBERS,
    MONTH([Post Date].CurrentMember.Member_Caption) = 2 And DAY([Post Date].CurrentMember.Member_Caption) = 29
)


Then I've amended the portion of the calculation which looks for the latest sale date (note the - Feb29):

ParallelPeriod(
    [Post Date].[Post Date].[Year],
    1,
    Tail(
        NonEmpty(
            Descendants(
                [Post Date].CurrentMember,
                ,
                Leaves
            ),
            Measures.[Total Price]
        ) - Feb29,
        1
    ).Item(0)
)


Thus when it looks for the latest sale date in, say, 2012, it will ignore Feb. 29, and compare to the previous year's sales from Jan. 1 to Feb. 28 instead. I'm still open to suggestions if there's a better way to do this (particularly that awful Filter expression I used).