One consultant I spoke with said they have had some luck using a JS script that includes code to write the tag to the DOM. Something like (Head.appendChild). Your milage may very with a meta tag, but I thought I would share incase you wanted to try.
After asking around a bit, it doesn't sound like injecting a meta tag would work as expected via any Tag Manager. The reasons is that the browser will likely have already parsed through all the meta tags before the tag manager is loaded and able to inject the meta tag.
It sounds like this tag might need to be hardcoded into the code rather that injected via tag manager.
According to the documentation here, any sequential HTML without the <script> tag is injected in the <body> instead of the <head> tag. Since I see your trying to deploy a <meta> tag, it doesn't make since to use a <script> tag.
Let me ask around internally and get back to you. Stay tuned.