You are exactly right to put the anniversary on them, so that people know you are not newlyweds. Otherwise, you will get a LOT of questions about when you got married, and people will be disappointed to hear it wasn't just a few days ago. I know - it is silly - but the other guests there really do ask; we got asked so many times I lost count. And some people seemed really surprised that we actually were newlyweds - apparently a lot of people use them who aren't, I guess to get special treatment. And a lot of people who didn't get married at WDW use the ears, too; we spoke to a CM who said he is disappointed when he thinks he sees a Disney couple, but when he asks where they were married, they say something like "West Virginia!" So I think that putting the years of your marriage is the more honest route to take, and it will avoid all those questions. "When did you get married?" "It's on the ears - 20 years ago"
We met a couple who was there for their 50th anniversary, and what they did was buy patches that had the number 50 on them, and had these patches attached to the ears. I don't think that anyone there will monogram onto the ears; in fact, I have heard of couples who wanted their names put on, and the place at WDW wouldn't do it.
Another route to take, besides the patch, would be to get some paint from a craft store, and stencils, and paint in the number 20 on the ears of the hats.
Cheers!
Heather