Is Rhino® lining a viable option for a residential roof? I know for many people it is cost prohibitive, are there any regulations concerning polyurethane coatings on a roof? Has this been done before?
To answer the question directly, polyurethane is used on metal roofs, but the physics behind all of the roofing products that exist is tailored to roofing requirements. There's lots of info online about polyurethane on metal roofing... suggesting that it doesn't "breath".
I have also considered rhino liner on asphalt and cedar shingles, but the cost is quite high and the possibility of condensation problems was too risky. But if you decide to experiment, then be mindful of the fact that roofs need to breath or else condensation will build up and rot the wood underneath. This is one of the reasons why plastic, steel, and/or other non-breathing roofing materials are corrugated (allowing air to flow under the panels).
I have used plastic sheets to temporarily cover unfinished roofs; after 24 hours the amount of condensation is amazing.
Various paints (and tar) are often sold for roofs, including aluminum, rubber, and acrylic latex. I have used tried all of them. Tar definitely works the best. Aluminum paint works too, but you will probably need to apply a few thick coats. Rubber works well temporarily, but decomposes quickly (after a month or three). I tried an acrylic latex paint on an aged and leaking roof on a wood shed. It helped reduce the leaking after 3 coats but overall, it wasn't effective for stopping the leaks.