How hard would it be for the government to issue an edict like "from 2025 all research must be published in an open access journal to receive public funding"?
In France it is more or less like that. You have to upload your publications to http://hal.archives-ouvertes.fr you want them to "count" (from the point of view of public research).
Forcing open access is still the same as just paying for the subscription, as far as it goes, since if public funds are used for this, it's all the same thing.
Maybe just bill them as an extra tax if they want to publish behind closed doors. Gradually increase the tax until they see the light. Or call it a fine if you like.