hg fixwhitespace you can either fix only modified files or all tracked files which is quite limited set of choice.
I think it would be the best to share file selection logic with
hg status which:
--alloption lists truly all files (not just tracked as
- have bunch of other useful display options, e.g.
- most useful: let you specify a particular folder/file you want know about
- quite useful: let you specify exclude/include patters.
My particular case: want to fix only files of a particular 100 files of a projects out of 10000 files solutions.