This is kind of a hack, but you can create a new registry by creating an IFRAME and registering elements through its document, for example:
<iframe id="registry2" src="javascript:"></iframe>
var iframe = document.querySelector('#registry2');
iframe.contentDocument.registerElement(...);
etc.
There are other ways, but they all depend on this one weird trick of creating a second, unrelated document and using its registry. Having the iframe is convenient (although heavier) because you can also run scripts in there, and the scripts get an isolated global object.
HTH,
Dominic