I have not heard of any 401(k) fund than invests in Bitcoin yet. Also I don't think there is a suitable investment vehicle that is available for regular (non-accredited) investors to buy in their rollover IRAs. However, it is possible to set up a trust that would allow you to hold bitcoins in an IRA.
I must say that a ira or a roth ira that is involved in bitcoin will surely fist all off enjoy a good constant positive growth. Now i personally have not heard of any institutions offering this service in the USA with a good track record. You mention something about a growing number of private companies offerr 401k. I say we just wait auntil more lol.