I have a request from a client to position the footnotes block 'hanging' a fixed space below the end of the text content of the page, rather than anchored to the bottom margin of the page.

Prince currently (v.11) appears to be doing the equivalent of

@footnotes { 
  float: bottom; 
}

I'd like the equivalent of this (assuming the footnotes block is treated like a standard block element that just happens to always be the last block element on any page):

@footnotes { 
  float: none;
  margin-top: 10mm 
}

Is there any way to do this?

Not yet, but it's an interesting idea. We need to think about how it interacts with page floats and other page content.