You could use the content type header with parameters as explained here
The spec seems to explicity prohibit this.
The section explaining the top level links structure does not specify any specific link names
No, but the only examples here are self and related; the section on links in relationships explicitly limits to self and related; and in this discussion one of the primary authors (@ethanresnick) seems to confirm that in v1.0 only self and related are permitted.
So my understanding is that currently the only permitted link values are self and related. 