Its actually quite simple. Have the IRS issue a "rule" that only treasuries count as tax delayed investments under a 401k. They have ruled on it before (to cover self directed plans, precious metals, etc), and could again. And whammo, your retirement contributions become govt IOU.
That I can actually see as being realistic. Which still wouldn't be direct confiscation. Unless you were forbidden from withdrawing out your funds, or given a ridiculously high withdraw penalty. Then yep, it'd be confiscation via creative accounting. Much like Social Security being a tax via creative accounting.
Credit unions are already have with capital requirement rules that essentially require them to buy government bonds. I could see the US government mandating more purchases of government bonds via creative accounting. Direct theft is a bit less likely.
Don't get me started on the MyRAs.
While it's always good to encourage people to invest in their retirements, the administration is way over-touting the MyRA. They're doing the hip marketing thing, acting like people putting twenty or a hundred bucks a year into an investment vehicle with a crappy return will see themselves sitting pretty in their retirement years. Nevermind the $15K cap to the thing, which in 30 years will probably be about three months living expenses for the average person. Some retirement.
What the hell is the point of a $15k max roth IRA with likely below inflation returns? Seriously, that has about no realistic value aside from a tertiary retirement vehicle.