It's done the exact same way as uploading a file with action=upload, except that the name of the parameter is 'xml', not 'file'. I've updated the documentation for import [1] and upload [2] to mention this.
[1] http://www.mediawiki.org/wiki/API:Edit_-_Import [2] http://www.mediawiki.org/wiki/API:Edit_-_Uploading_files
Thanks, finally got it working! I've updated the Import page with an actual code example.
Cheers, -jani