They've got ham radio plates. Like most other states they look just like the standard plate except they have a ham radio callsign instead of something in the format of their normal sequence.

So I guess it will come down to what is the smallest subset of veterans that gets a special plate in every state?

Here in Washington there isn't actually a veterans plate. There are 6 military plates (army, marines, navy, air force, coast guard, and Washington national guard) and a gold star plate.

You also get a decal to indicate status (veteran, retired, active duty, reserves, disabled veteran, army guard, air guard, army retired (for Army National Guard retirees), air retired (like army retired but for Air National Guard), fallen hero (for family members of someone killed in action), family, and gold star.

I'd say a military plate plus the state issued decal that identifies you as a veteran would count as a veteran's plate.

If it is just "veteran" that gets a plate in every state that would be something like 16 million people, which is more than ham radio license holders (under a million).

It is starting to look like there might only be two groups that have plates in all states: veterans and ham radio license holders.