I submitted a patch for random sort in SMW. It has been applied in r43156. (see https://bugzilla.wikimedia.org/show_bug.cgi?id=14978)
Unfortunately, there isn't any documentation for it on the SMW website and wiki account creation doesn't seam to be open to everyone.
It could be also great to add it in the release note.

Basicaly it adds a new sort order which can be "rand" or "random"
And it can be enabled/disabled by truning $smwgQRandSortingSupport on/off.