It should not be hard to remove the bolt as all the tension is relieved once the bolt snaps. Also, perhaps it's just the photo, but the crank bolt looks hallow; you have already drilled it for an "easy out"?
Presuming you snapped the self extracting 7mm allen bolt, I would not worry about a dust cover; just use one of these.



Also, since I have the Campy tool, I have not had to try and remove the self extracting ring with a spanner, but there are some adjustable spanners that I'm sure would do the job. Finally, all this is fine until your customer needs the crank arm removed for some purpose. He will either need a Campy reverse thread crank puller, which are still quite available or remove the self extractor ring/bolt from the drive side and use it on the left.