You're not talking about forcing participants to self-direct; you already have that, but presumably there is a default under the existing platform for those who don't make an election.
So, you, the idiot sponsor, or someone else, needs to figure out what to do with accounts for participants who do not make an election. You can not, literally, force someone to direct their account.
What is proposed is not a good idea, to understate the case. Sponsors think that if they let everyone do whatever they want, they avoid liability. Not so.