You could do that with addmetatag from httprequesthandler afaik.

Yes but you would have to define it per screen. You have no way to define general metatags to be used across all screens.

As a work-around, you can use a WebBlock to add your metatags and use that WebBlock in every screen...

In version 6.0 all screens have a "Master Page" (a webblock with placeholders). You are able to define the webblock that is used by default in the theme.

You can place the metatag by using the HTTPRequestHandler in the preparation of the MasterPage webblock and it will be reused in all your screens that use that masterpage