As I see you can link the keyword extension to one of your own python scripts that extends: [keyword.py](http://mercurial.selenic.com/wiki/KeywordPlan?action=AttachFile&do=view&target=keyword.py) to transfer it to svn formatting. I think maybe this is the easiest. However it seams to be incorporated: [newest branch of keyword](http://www.blacktrash.org/hg/hgkeyword/file/a0451f80f665/hgkw/keyword.py), see line 67.
Try `{date|svnisodate}`.