I frequently travel abroad and it's not always feasible for me to get to a US consulate/embassy to notarize my closing documents in person as required by the HMLs I'm aware of.

I want to emphasize the 1% rule is just a quick and dirty way to evaluate the economic feasibility of a perspective deal.  

Otherwise you're stuck with a renter's policy and coverage for contents only.From a business perspective the zoning alone takes the feasibility off the table unless you're in a very specialized market that already has the zoning to allow them in more areas.  

ADU rules vary widely by locale in Pinellas so you'll definitely want to research where it is most feasible. 

My question is: How much equity should an owner possess in a property to make an owner-financed deal both feasible and beneficial?

It may not be feasible always feasible to pass on this suddenly increased costs to tenants, in these situations would it make sense to sell and move to a market with lower property taxes?

Could it be feasible to refi at a higher amount as a rental?

Land banks are good for one thing:  redeveloping property that is not economically feasible for an investor--i.e., the numbers don't work. 

Larger is not the goal, but evidently max leverage on high end homes and loans is very feasible if you search hard enough, and qualify.

That will dictate, to some degree, which investments are feasible for you, right now.