This idea has been discussed before. No DSM owner is very enthusiastic about having their engine wrecked by such a minor event as a broken balance shaft belt.
Unfortunately, the consensus was that there was not enough room between the belts to provide a strong enough shield. The shield would have to be so thin that the belt would likely tear right through it, causing it to contribute to the problem rather than solve it.
Those who are interested in such protection may wish to remove the balance shafts completely. See this section of the 1000AAQ and theFAQ Locator for the latest information.