Currently it's not supported, please file enhancement request with use case.
Thank you for that information.
I filed an enhancement (22962680)
I also found a way doing it by creating a custom TVML Element and Custom TVML Style.
There is a little Objective-C code needed...
And read more about it: https://medium.com/shopgate-mobile-commerce/hacking-tvml-4387e65a9b94#.dos8q646t