SMIL extensibility?

The question of whether there’s any way to include installation-specific data in SMIL has been bugging us at HUL. It appears from the SMIL 3 documentation that its W3C authors intended that any version of SMIL that incorporates non-SMIL elements should define its own DTD. But as soon as you do that, it’s not interoperable with conformant SMIL files that specify the standard DTD.

Most metadata schemas provide some way to include data in a separate namespace, which readers can use or ignore at their option. The lack of this provision is a serious limitation. Of course, it’s always possible to do something like creating a METS wrapper that has SMIL and other stuff in it, but that’s not an optimal solution.

Has anyone figured out a way to deal with this issue?