The restriction on the SVG title element has been clarified and loosened slightly so that any HTML elements can be used in it (but only HTML elements).

Epubcheck's implementation of the older restriction to phrasing content isn't working as expected right now, regardless. You can't mix text and markup. If the title isn't purely text, then all the content has to be inside an html element for epubcheck not to throw an error.

In other words, this is valid:

<title><span xmlns="http://www.w3.org/1999/xhtml"> ... </span></title>

But this causes errors:

<title>foo <em xmlns="http://www.w3.org/1999/xhtml">bar</em> baz</title>

So the changes needed are to allow any html elements and also allow mixed text and element content.

See w3c/epub-specs#2358