How to use multiple Vocabularies with HTML5 Microdata (different dictionaries than schema.org)

Let's say I have this extended HTML snippet with extended data:

<div itemscope itemtype="http://schema.org/Corporation">
  <div itemprop="legalName">Example</div>
</div>

(I tested this with the Google Structured Data Testing Tool.)

And now I want to add a property that is not available in schema.org, but in another vocab (in this case, W3C registered Org Vocab [1]):

<div itemscope itemtype="http://schema.org/Corporation">
  <div itemprop="legalName">Example</div>
  <div itemprop="http://www.w3.org/ns/regorg#registration">XXX</div>
</div>

At a minimum, google claims:

Error: Page contains property "http://www.w3.org/ns/regorg#registration" which is not part of the schema.

I could delete itemtype, but I do not want this, because this is a loss of loyalty [2] Is there no other way to do this? With RDFa, this is not a problem.

EDIT

I found [3] that offers duplicate content, but this is obviously not a good solution.

[1] http://www.w3.org/TR/vocab-regorg/#ref_rov_registration

[2] Using itemprop without an item

[3] http://www.w3.org/wiki/Mixing_HTML_Data_Formats#Mixing_Vocabularies_using_Repeated_Content