My argument was based around that the current system is designed as a directory of opaque objects and so the extended attributes should be kept opaque to the kernel as well and left open to interpretation by userland. What I am most unclear about is under which circumstances is this backchannel communication preferable to passing the extra information over the IPC that needs to be performed anyway in order to open a surface.
That's the part I had trouble with as well. Passing the blob through the kernel saves a little IPC but also seems unnecessary, and so rubs against my kernel minimalist side...
Passing the blob through the kernel does give you a basis for more complex security since you can agree a blob format with the kernel security layer and add security hooks to the interface in future.