Hypothetical situation:

Client is a 45 year old W-2 employee who’s income is over Roth IRA contribution limits.  They have a 401(k) at their job and they also have a traditional IRA with $100k in pre-tax dollars.  Let’s say we wanted to start doing some backdoor Roth contributions to have a different tax bucket for retirement.  The pro rata rule obviously throws a little wrench into that plan.  Assume that either we can’t roll the $100k IRA into his employer 401(k) or we didn’t want to because of poor investment options.  The client does not have any side income reported on Schedule C.  Could you still open a Solo 401(k) under the client’s SSN as a sole proprietor and roll the $100k IRA in the Solo 401(k) to avoid the pro rata rule?  Employee would obviously not make any other contributions to the 401(k) since they don’t have any Schedule C income.

Is there anything that I’m missing or would that be a valid planning opportunity?  The potential issue that popped into my head is that if the client were to get audited then the IRS could potentially argue substance over form.  I would love to hear your opinions.  Thanks!