I've written a very small plugin, inspired by Can ikiwiki be configured as multi user blog? which adds match_userdir to IkiWiki::PageSpec
userdir() in a PageSpec takes no arguments and checks if the page is the users userpage or a SubPage of it.
The plugin is named userdirps.
In case my gitbranch template is wrong: https://github.com/bfree/ikiwiki/commit/1f699d15c0778702cc5264dd3c8f822c6b0609ba