I figured out how to get your tumblr tags to display as meta keywords on the permalink pages:

<meta name="keywords" content="{block:Permalink}{block:Posts}{block:Tags}{Tag},{/block:Tags}{/block:Posts}Permanent Tag{/block:Permalink}" />

Where it says “permanent tag” in there, you can replace that with a tag that you want to show up on every single entry, or you can erase it. The end result of copy pasting this code into the head of your custom tumblr theme is that it will create a meta keyword for each tag, pretty neat.