Sounds fun, but everyone solves the same problems (ok, ideally not: "sets of custom-built, rapid fire coding challenges built by each employer. You can use Hackermeter’s pre-provided challenges, or build your own")? How long until copied solutions corrupt the scores? Better check performance on a held-out set before interviewing/hiring.
Let's not get into plagiarism-detection arms race; consider automatic refactorings / pretty printers with variations, and human input to vary variable names.