One of the groups interested in the pluggable file system work says:

This is the case that maps well to SSD trim, which is in our API today. Basically you trim that region, which deallocates the underlying physical space and gives you zeroes if you read it afterward. Aside from freeing up space, the more important effect is reducing write-amplification since our garbage collector knows that when we pick up the block containing the trimmed data, that data need not be moved to a new block.

WiredTiger should make a trim call when we "free" a data block, at that point we won't read the block until after it's written again. This is a pretty common occurrence, because we're a no-overwrite store we never overwrite a block that's in use, we write a new block and eventually free the original for re-use.