The short answer is that you can't only make a leaf-level category multiselect. Only the entire dimension can be designated as multi-select.
One option would be to have the leaf level values in a separate dimension and have that as multiselect with a precedence rule controlling when to show it.
Alternatively, you can make the entire dimension multiselect, but this turns out to be very cumbersome, and I don't believe this is supported by the Assembler application. The way this would work (and you can test this in the orange Endeca JSPREF) is using a combination of the N= and Ne= parameters.
If you clicked the first level of a hierarchy, N=<dimvalid> and Ne=<dimvalid> . If you don't set the Ne= value, clicking the first level of the hierarchy would just allow you to click the other values in that first level (because of multiselect). Putting the Ne= in the URL helps you "descend" to the next level.
This isn't really a supported way of navigating, and thus it's not really supported by the Assembler framework.