And Trump can definitely order his agencies to re-write those rules.

So his IRS could alter whatever rules are in place to allow it. I don't know the legislation that created 401Ks, but I had assumed (possibly incorrectly) that Congress laid out some very specific rules. I vaguely recall (again, possibly incorrectly) that the 401K creation wasn't a slam-dunk, and one side was demanding some pretty tight guardrails on it to allow it to pass. I would think that the IRS would have no discretion about those rules, only rules that they were allowed to impose after it was law.

I only ever worked for one company with a 401K, so I really have no idea what is out there. Ours was through Fido, and even then, only maybe a dozen options were available for fund choices. As opposed to an IRA, where you can declare pretty much anything an IRA. Within that, I'm sure one could trade crypto today. The 401K was a special creation.