W3C DTD allowing attribute 'ecmaxmltype' of <data> ?

Is this attribute of 'data' proprietary or part of some proto spec that
might have an endorsed DTD?

I'd like to use this attribute in my apps, and it works on some platforms
(e.g. Voxeo), but it's not part of http://www.w3.org/TR/voicexml21/vxml.dtd

I'd like to use such a DTD so doc validation succeeds, without having to
create my own DTD fork.

Cheers
David