Excellent question, but this should be preventable.
First of all, if you're treating JUST the crow's feet, which are the wrinkles/lines on the sides of the eyes, then you (meaning your physician) should not need to inject any Dysport in the regions that are most likely to CAUSE droopy eyes. That is usually the forehead area. Over enthusiastic injection and dosing of the forehead lines is responsible for the bulk of droopy eyelid issues that can result from these injections.
So the best treatment is - prevention. In this case, that means that you should start with small amounts of Dysport, and limit it to the regions on the sides of the eyes. You can always get a little more but in the event that you're unhappy with the result, you really have to just wait it out (ie, let it absorb slowly on its own over a several month period).
And going to an experienced physician, and being sure to explain what you're interested in and what you don't want, is the best way to start this process.
I hope that this helps, and good luck,
Dr. E