This has enough uses and adds little enough in the way of syntactic complexity that I believe its inclusion in core CommonMark is reasonable.
I tend towards making it a modified form of the link syntax, since anchor links are links, and because it would allow anchoring to any location in a document, not just to a limited number of elements. (I believe that Markdown Extra doesn’t allow arbitrary placement of anchor links, though I may be wrong.) That said, I wouldn’t be against a standard syntax that allows the setting of arbitrary attributes on an inline or block element, which would serve the same purpose but allow greater flexibility.
That said, in the end I have the feeling that this is going to be deemed “extension material” and left for other people to implement.