Such extension is very simple to make. You register a tag extension, and on parse you return something. However, you store the parsed content in page_props. You could also store it in ParserOutput, page_props seems cleaner, although it would be unstructured data, anyway. What kind of contents would the tags have? An name per line? What if there should be something besides the name (eg. a url or email)?
Note that you can achieve a similar functionality right now with a (series of) template which you place at the bottom of the page.