JSDL has shown that extensions work fine even (sigh) with XML namespaces, without breaking interop.
Leaving extension points with xsd:any with namespace #other and lax content allows for easy interop by just ignoring vendor-specific stuff.
However you slice it, namespaces are almost certainly going to be a necessary evil for extensibility. Even for our own stuff namespaces are useful for differentiating between resource types rather than throwing everything in one bucket. I've already seen a "vx_au.net.aos" namespace option proposed for JSON extensibility based on a similar monstrosity I suggested earlier for XML - at the end of the day all markup ends up looking like XML anyway.
Also playing nice with other standards includes OGF standards of course... is it safe to say these are mostly/soley done in XML?