How to open a page in Chrome from the command line, in a new tab or an existing tab as appropriate?
Solution 1:
This is a simpler command line, but will still open a new tab:
open -a Google\ Chrome <file>
Solution 2:
Turns out this can be done with a few lines of javascript in a Chrome extension.
Still open to other answers, but in the meantime I've written a Chrome extension to do this and put the source online.