We can see all the changesets and the files involved using
hg outgoing -v
but the filenames are all scattered in the list of changesets.
I usually use
hg outgoing -v | grep files
It makes the listing shorter, but doesnt sort. But thus far I havent been in a situation where I want to push so much (and at the same time check the files) that its been a problem.
[Edit] To do what you want:
cut to remove the files: parttr to put them on separate linessortLike so:
hg outgoing -v |grep files: |cut -c 14- |tr ' ' '\n' |sort -u
You can put this in ~/outgoingfiles.sh or something to have it nice and ready.