ThorsHammer -
If you want to use compressed air to clean the cones on the midwoofers, use some making tape to tape off the ribbon tweeters before you do. With a couple of layers of tape to protect the ribbon element, you should be OK. But certainly don't use it on the tweeters! I wouldn't try to blow them off at all and would just leave them alone.
If you feel the ribbons are not performing well due to the dust, you could always have new ribbon elements installed. The ribbon element is the only moving part and can be replaced. If you feel the need to do this, let me know.
- Jim