No need to seize anything. Deploy a Government Mastodon instance and run a PSA informing the public that the Mastodon is the official channel for emergency communications.
Germanys government has a own mastodon instance up and running. And while different institutions moved away from Twitter over there, citizen do not follow because "there is nobody using mastodon I know" or "mastodon is too complicated"
People don't need to use it as their micro-blogging platform, they just need to know that a site like govern.xy is the authoritative source of alerts. Mastodon's default timeline for non-logged visitors is simple enough for visitors to understand and get the information they need.