How hard is it to properly owner-finance a home that's owned outright (in a way that legally protects both seller and buyer)? What resources would be a good place to get an idea of how this would be done?
Your best bet is to start by surfing www.CREOnline.com
That's where the creative real estate folks hang out.
After you get well oriented with how sellr financing is done, you'll want to hook up with a local creative real estate attorney to review the documentation you structure.
You do not really need a lot of creativity to create a note that is held by the seller, with all the regular terms for a note holder to foreclose.
I have done it a couple times, and the standard secondary market note & deed of trust/mortgage note has plenty of protections.
You are likely to need a local RE attorney to draw up the note.
If you want to change anything state law regulates note terms also and can be a real mine field for things like foreclosure terms.