I just flattened a piece of bamboo this afternoon on my belt sander. I usually just hand plane it flat because the deflexing causes a problem, but I modified my sander and was able to get it done easily. Took 15 minutes tops.
In the picture you can see a piece of white plastic I attached to the dust collector port on the sander:
It's really just a homemade miter fence for my tablesaw. The white part goes in the groove on the table. It allows you to flex the bamboo so that only one section touches the sander bed at a time.
Here's a pic of how I used it:
With the push block holding the bamboo to the sanding belt, the miter guide keeping the bamboo propped up, and me flexing the back of the bamboo up off the sander, I was able to control just where I wanted to remove the material. The bamboo only touched under the push block, and I didn't sand off any skin...... this time.