If your space is small, there are willow that don't get massive. I know, the first image that came sweeping into my head was "fence made of weeping willow breaking up sidewalk, growing into 2 foot thick solid wall of yucky rambling mess.....Aaah"
And then I remembered there are many different species in the salix genus that would stay well behaved and look nice if you planned for the mature trunk size. The first two that come to mind are dappled willow (salix hakuro nishiki) and blue artic willow (salix purpurea 'Nana'). Not to mention, why limit yourself to willow? Use fruit trees, flowering shrubs like weigela or quince, even woody hydrangea...there are so many options that would work! OOooh....cotoneaster would work great! Especially cotoneaster salicifolius! It has a "willow-like" leaf, long straight branching, pretty white flowers/red berries that follow, is long lived, quite hardy, non-suckering, and the trunks wouldn't thicken so much that you would loose definition.
Thank you for starting this thread, it has really gotten my juices flowing! I've been wanting to sculpt with living plants for a while....this was great eye-candy! Share pics if you start something!!!