I love the idea - I think that it is much harder to accomplish something great with less than it is to use a Mahlerian style orchestra. The sense of accomplishment that you get from making a great piece of music out of fewer instruments is rewarding.
I personally refuse to place percussion in any concerto of mine, as I hope to give the soloist and orchestra the driving pulse. One example, my Piano Concerto No. 1, was scored for a small orchestra and I believe that I accomplished quite a bit with it. Reaching that small ensemble's fullest potential was one of my goals. Here is the scoring for the piece:
2 Flutes
2 Clarinets
2 French Horns
Solo Piano
Orchestral Strings
If you are curious about the work:
http://www.jbelelieu.com/work-piano_concerto_no1.shtml