I don't have a solution but I do have vinca.... my non poisonous snakes love the stuff.

what about really thick matting groundcovers like wooley thyme that spreads extremly fast and other thymes. great weed contolers too. I don't know much about copperheads but I believe they are too thick to hide under things like that.
My snakes love rock piles and branch piles and empty pots, anything they can get under.

Good luck. I will be interested to know what other people say.

The low ground cover will attract snakes. How much do you trust your neighbor's identification? There are many snakes that people mistakenly identify as copperheads. The mountains of Northwestern Maryland are the main suitable habitat in Maryland. I suppose its possible they could be in the outer suburbs of DC. They can do o.k. in suburban areas if the habitat is suitable.

I looked into some pretty specific county level snake data and it appears that many of the DC area counties do support a few small isolated copperhead populations, so I retract my skepticism of your neighbor's id of the snakes (Looks like I need to go looking more the next time I'm visiting relatives in Montgomery County)

Unfortunately I can't think of any vegetation that won't be a potentially attractive place for snakes. Even if you plant nothing at all, nature holds no guarantees so it's a matter of how important a slight reduction in risk of snakebite from very low to very very low is and how important having an attractive natural yard is.

I've always heard marigolds keep snakes away. Doesn't appear to be any empirical proof to back this up, but for anecdotes, we never had snakes when marigolds were planted. After we had some come around our pool, my mom had me plant them all around. No more snakes.