How to make a plaque
Select sign
Place on exhibit wall
Type a memorial
done
Yes, sure we could do it that way

but it would be a sign you probably use for other things in the zoo too and they wouldn't look like a memorial plaque.
We probably didn't see every sign that will be in the game by release - but the signs I saw, would be to big to fit that purpose... for example to fit on a fountain or statue.
It would be useful, if we had a option to change the size of a sign to bigger or smaller. Not only for this purpose.
We could use a sign we like, and make it smaller independently to use it as a way sign or even to make a animal head sign bigger to fit better on the esthetic of a building.
Just a thought, but I think a helpful one.