How about something like this for the toolbar, in /plugins/toolbars/_quicktags.toolbar.php, which would add a new button labeled 'SEO'?
This example drops both blocks into the post at once with no need for a closing tag.
There have always been line-wrapping issues with script blocks in the _quicktags.toolbar.php file, especially when copying a sample block from a forum post, so the best/safest option might be to just open the file and copy an existing block to maintain the line-wraps in your particular editor, then replace the content of your copied block with the info from this one... Maybe?
Anyway, this block should just dump the pre-formatted starting and ending tags into the post when you press the 'SEO' button, and then you replace the CAPITAL text with your own description and keywords.
I guess separate buttons could be made for the description 'DESC' and for keywords 'KEYW'... This example drops the opening section of the block into the post, you make your edits, then you hit the button again to close the tag block. It creates a separate button for "description" and for "keywords".
but some of us have already cluttered their admin toolbar buttons with other custom stuff and space can become an issue after a while so the single-button option might be desirable.