Seen this:
https://forums.modx.com/thread/72386/getpage-cannot-get-offset-and-total-to-work
Which suggests what I want isn't possible (even though detailed in the docs?).
Have seen this as well:
http://www.markhamstra.com/modx-blog/2011/12/preparing-custom-snippets-for-getpage/
Which suggests something might be possible, but only through custom snippets. Hm.