Sure you can engrave it. Odds are you will go through the "fill" which is the silver and down into the base metal...
Depending on what the base metal is, is going to determine what the final result looks like. And whatever it is will probably blacken over time. If the customer can live with that?
If it is a "modern" piece it may also be clear laquered or have a transparent electroplating applied to reduce tarnish. Engraving will remove that...
you'll possibly encounter a problem when cutting thru plating. such is so thin, sometimes it will cause flakes to form around the cut line. this can happen to more expensive items as well as the cheaper ones. if this happens, theres no fix for this other than removing all the plating and going from there.
Our U.S. quarters are good examples of filled or laminated materials.
In its infinite wisdom, our fed. govt. could not simply call our quarters laminated, therefore the name filled was thought up after a research committee spent millions of our taxpayer dollars to determine that filled conotates fullness while laminated insinuates cheapness, such as plywood.
Besides, they had to find a new way of dispatching the excess B.S. that Washington generates and what better way than to "fill" a worthless coin with the excess B.S.!