After a lot of time, we have just found a good solution to repair your scratched HP41C screen. There is an abrasive product used to repair watches' plastic screens, called Poliwatch. You can easily get it from Amazon and other sources.
Here is the link to the product in Amazon:
The method: just apply a little quantity on the scratch, and then rub with a soft cloth (looong method), or use the cotton-felt tools of a dremel-type machine. This gives a bright finish in a very short time.
Beware: do not try to use this material on crystal - it is so hard that it would not help at all.