Can microdata be implemented in meta tags?

Your plan of using meta data for microdata is not viable. Here is Google's FAQ about why it isn't showing your data in the search results:

Is your marked-up content hidden from users?

In general, Google won't display any content in rich snippets that is not visible to human user. Don't hide the content that you have marked up for rich snippets using techniques like display:none, value-title, or css. Google will ignore content that isn't visible to human users, so you should mark up the text that visitors will see on your web pages.

The only way to get Google to use the microdata that you supply is to mark it up where it lays in the page, visible to the user.

At this point, Google is not penalizing for trying to abuse rich snippets other than just turning off rich snippets for that site. It wouldn't surprise me if Google were to start excluding sites from the search results entirely when Google finds the site trying to use microdata in a way that doesn't corform to the guidelines.

As long as your meta data that you are marking up is also visible somewhere on the page, Google is unlikely to penalize your site as malicious. However, their automatic tools detect when you are marking the data up in a non-visible location and they won't show it in the search results.

Using meta (and link) elements for Microdata is fine. Sometimes there is even no sensible alternative to it, e.g., if specific codes have to be provided where it would make no sense to show them to your users.

Google even uses meta in some of their Rich Snippets examples:

• Products and Software Apps:

  <meta itemprop="priceCurrency" content="USD" />
  

• Reviews:

  <meta itemprop="datePublished" content="2006-05-04">
  <meta itemprop="bestRating" content="10"/>
  <meta itemprop="worstRating" content="1"/>
  

• Videos:

  <meta itemprop="uploadDate" content="2015-02-05T08:00:00+08:00"/>
  <meta itemprop="duration" content="PT1M33S" />
  <meta itemprop="interactionCount" content="2347" />
  

• Articles:

  <meta itemprop="datePublished" content="2015-02-05T08:00:00+08:00"/>
  

So the question is, how much is too much (if there is a limit at all)? And I think it’s safe to assume that there is no hard limit, it most likely depends on various additional factors.

However, it would make sense for Google not to dismiss Microdata markup if only meta/link is used. Why? Because they also support (and sometimes even recommend) JSON-LD for providing Schema.org data, and this consists only of "hidden" content (namely, a hidden script element used as data block).

And this would be what I’d suggest in your case: If you don’t want to add the structured data by marking up your existing elements, use JSON-LD.