Ashley 
sqiddster
So I found a way around this, at least for published versions. Try publishing the capx above and then before the closing head tag (in index.html) add:
<script>
WebFontConfig = {
custom: { families: ['Conv_VAGRundschriftD'],
urls: ['fonts.css']
}
};
(function() {
var wf = document.createElement('script');
wf.src = ('https:' == document.location.protocol ? 'https' : 'http') +
'://ajax.googleapis.com/ajax/libs/webfont/1/webfont.js';
wf.type = 'text/javascript';
wf.async = 'true';
var s = document.getElementsByTagName('script')[0];
s.parentNode.insertBefore(wf, s);
})();
</script>
This seems to force the font to load and all works fine. However, while that works for published games it doesn't help during preview mode (just because it's not as easy to edit the files).
Test page using this technique
Using the above code (with the "families" property changed to my own font family) I was able to publish my game and get it running. It still doesn't really answer why it's not working in the first place though. What technique is C2 using to load webfonts at the moment?