What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

Specifically, I am looking at partnering with an individual who owns securities (stock and mutual funds) having them take out a line of credit against their portfolio (SBLOC) at a low interest rate, and using that to purchase properties rather than using a mortgage.

You are still going to get the best rate possible with your significant down payment and with good credit I'm assuming.   

Perform the same task in each app to ensure a fair comparison.Load a property with pictures and details.Market that property.See what your marketing looks like from the public's perspective.Submit a fake application to see how easy the process is.Run a credit/screening report on yourself.Enter a maintenance request, assign a vendor, and attach a fake invoice.Enter charges to the tenant's ledger.Enter recurring charges and automatic late fees.Sign documents electronically.Run owner reports.After testing a few apps, one should clearly stand out.

But somebody could use a credit card fund the immediate need and then pay it back with the policy loan fairly quickly.

Caution:  Personal credit card financing is a huge risk and good luck finding 0 percent interest today.

Next Step: With with sparkies quote, I'd get that quote over to sellers and with an amendment all written up ready for there signing giving me a credit at closing for exactly that amount. 

Bank Credit Line How did you add value to the deal?

The best scenario for the buyer buying subject to is to have enough money and good enough credit to qualify for a new loan at current rates should the due on sale clause be invoked by the lender  

But without the experience, knowledge and capital (actually CREDIT CAPACITY) I accumulated the prior 2 + years, this transaction would not have been possible.