Why aren't custom tags supported yet in Selenium WebDriver? I realise the W3C web components proposal is still under review, but browsers are already implementing them widely (angular apps come to mind e.g.
You can find custom elements by using the tagName;
Example of how to find an element that looks like this:
WebElement frame = driver.findElement(By.tagName("iframe"));
Full documentation is available on the Selenium API
I would also suggest you maintain a separate list (or Enum) of your custom elements as they are dynamically generated so easily changed in the code base which could lead to a high number of tests needing updating.