Actually, looking at Schwab's "individual 401k", it looks to me
like you can contribute the first $15,000 of Schedule C income
Now, this is only a regular 401K, not Roth 401K, but as someone
pointed out in another thread, you can use the headroom in AGI
(generated by deferring to this individual 401K) to do a Roth
conversion. So, if I'm reading this correctly, you can effectively
put all the deferred money into your Roth ! Seems almost too
good to be true ...