Did you mean
z(:, iPos)
as it's a bit unusual to use a logical index as a linear index when it's smaller than the number of elements in the matrix.
To answer your question, unfortunately, you can't index into temporaries in matlab, be it the result of an expression as in your example, or the return value of function. You always have to create a variable to store the result and there is no trick that can work around that.
However, in your case, you could do
z = y(iPos) * x(1);
Or if you meant to get all rows:
z = y(iPos) * x;
