Just as you said, I'd recommend weights. Percentages would be incredibly useful (don't know why they aren't supported), but one way you could do it is like so:
<LinearLayout
android:layout_height="fill_parent"
android:layout_width="fill_parent"
>
<LinearLayout
android:layout_height="0dp"
android:layout_width="fill_parent"
android:layout_weight="1"
>
</LinearLayout>
<View
android:layout_height="0dp"
android:layout_width="fill_parent"
android:layout_weight="1"
/>
</LinearLayout>
The takeaway being that you have an empty View that will take up the remaining space. Not ideal, but it does what you're looking for.