I think that SIMD code should not be written by hand but rather in a high-level language and so dealing with tail becomes a compiler's and not a programmer's problem. Or people still prefer to write assembly be hand? It seems to be so judging by the code you link.

What I wanted is to write code in a more high-level language like this. For example, to compute a scalar product of a and b you write:

    1..n | a[$1] * b[$1] | sum

    x = sum for i in 1 .. n: a[i] * b[i]