There’s a difference between speech and action. Government can say whatever it wants so long as its actions are consistent with law.
It’s really illustrative to think about how the design of our system applies to this kind of issue. For good reasons we require challenges to come with a demonstration of real, definite harm, and one reason is especially relevant here: the requirement of harm is one protection of freedom of speech. Authorities can’t be policing opinion and expression under that policy: they’re restrained to prosecuting things that are actually harmful, leaving ideas and philosophy free to be explored.
Anyway, to talk about whether a state can issue a plate condoning a law we have to ask, where’s the harm to be prosecuted? Who will stand up and present harm for our system to address? At least in the Texas case a driver can rightly or wrongly imagine that his ability to express himself is interfered with, but that doesn’t apply to the hypothetical here.
So long as a state is acting lawfully there seems to be no reason why it can’t also issue a plate criticizing the laws it finds itself forced to obey. In fact, that seems like a pretty legit method for the state to protest what it sees as bad federal policy. Imagine states issuing plates critical of federal immigration policy or drug policy as examples of not-that-far-out possibilities. DC’s “taxation without representation” plate is close to that line already.