History says that you can do twice that in your average mutual fund (6%).

This is SUCH a good question and often a stumbling block for Solo 401k investors so great to get expert help like you've gotten here from Brian, Carl and Dmitry.You can use almost any brokerage account with your Solo 401k including:Schwab: they call it a Company Retirement Account as Dmitry mentionedFidelity: non-prototype retirement accountTD Ameritrade: trust accountRaymond Jones: varies from branch to branch, but usually called a Master accountE*TradeVanguard: they have different accounts for mutual funds or ETFs/equities but definitely availableInteractive Brokers: Trust accountFor banks, we've found the following are familiar with/supportive of Solo 401ks:Titan Bank: FDIC insured, based out of Texas, clients in all 50 states, free checks, free debit card, no monthly fees if you keep $1500 in the account, online banking, etcSolera Bank: like Brian recommended, FDIC insured, based out of Colorado, clients in all 50 states, free checks, free debit card, no monthly fees (for certain clients), online banking, etcTD Bank: a particular branch in New Jersey (strangely enough!)

Liberty Mutual/Safeco has a program like this.   

I’m not concerned about recording since the Beneficial owner is still me.  

We happen to live next door in a SFH, which makes it “mostly” beneficial.

Looks like you're not married, so I'd think long and hard before placing somebody else's name on a mutual deed for a house that might not fit your financial plans.

This could be beneficial if he pays into the plan but not likely will and only stall for another 6 months after confirmation, at which point you can get a Motion to Dismiss or Relief and resume foreclosure.  

If so, what is the process and is it beneficial?

., if I walk away with $275k, I'd likely put it in mutual funds and bonds, perhaps VWIAX (mix of stocks and bonds), which returned on average 6.5%/yr since inception in 2001.If I refi, how does it work that I take out the current going price for the condo? 

It would be beneficial for me to pay off a 20% interest loan so that I do not have that expense and can therefore keep more income.I understand your point, that if someone has a 5% fixed rate 30 year mortgage, then yes carry that full term and use your money elsewhere.