S Chapter. Commercial Rentals. No Payroll. QBI

Is there anything stopping IRS from recharacterizing the activity of a rental property s-corp s/h? for purposes of calculating QBI?"

No, IRS has authority to recharacterize distributions into wages subject to FICA( Joeseph Radike vs US(CA-7 895 F2d 1196).
In this case they could argue that a reasonable salary would be the cost of hiring a Property Manager who typcially may charge 8-10% of gross rents.

Rather than expose your client to the risk of recharacterization with subsequent employment tax and loss of QBI, I would look into converting the S corp to an LLC(or SMLCC) where the statutory ground is much firmer and generally a much better choice for holding rental proeprties.

What about the social security rules. Social Security does not consider rental income to be earned income.

Services rendered by an officer or employee of a corporation are considered to be liable for social security and Medicare.

For rental property how do you compute UBIA cost basis limitation of 2 1/2% ? ( in reference to CaptCook post)
Is it the basis for section 199A gross basis of the asset minus accumulated depreciation or is the deduction based the gross cost basis ?
If it is based upon net basis after depreciation then a real estate entity that has no W-2 wages and a very low net basis would receive a very small deduction if above the 315k income level.