One way of achieving that would be to add additional attributes (for example of text line or text block type) to the classes that you want to have the meta tags for. Then you would have to alter the meta head template in order to be able to display different class combinations. This doesn't require extending, just use of built-in content engine.
BTW. There's a keywords datatype, but I don't think it's a good idea to use that for meta keywords attribute, even thought that would work. The reason is that there's more to that datatype than just collecting data. Keywords would be quite useful for tagging the pages.
