Good point, surprisingly few people submitted entries for stage 2.
I only submitted a simple pre-trained VGG model with bounding box regression on the annotations. Nothing more sophisticated because I had no clue how to do good validation to gauge my results. I think a lot of people felt like they were stumbling around in the dark. This has been a really good lesson for me though, and I'm going to pay a lot of attention to getting it right in the future!
This thread has a nice discussion of some validation strategies.