The principles that are the theme of the thread, if I'm not mistaken, is
if you are required to pay taxes on your WMO, and
how you pay those taxes. While it is true that many pay those taxes at the time of their annual filing, others have to pay monthly.
Normal people pay their road taxes on fuel at the gas station at the time of purchase. No need to file anything; it's automatically taken care of, just like sales tax. In other words, it is the seller's responsibility to file and forward your tax payment on to the government.
If you are planning on using WMO in an on-road vehicle, pertinent issues to discuss with your tax authority are: What is the source of your waste oil? has tax been paid previously on it? (sales tax on the purchase of "new" motor oil) Have you ever purchased your fuel, or is it being donated to you? What is the proportion of WMO that you plan to use with relation to regularly taxed, right-out-the-pump fuel?
When I phoned my state to set things straight ahead of time, there was confusion on the part of the rep if I was using a small amount of used oil, such as the amounts of off-the-shelf fuel additives, or a more significant portion. I believe their reasoning was that additives such as STP, Lucasoil, 9-1-1 etc haven't been assessed road tax due to firstly their prohibitive cost, and secondly their intended purpose. You may avoid road tax by filling up on pure STP, but you will be paying a bunch of money in your product purchase to avoid road tax, and will still end up paying sales tax.
You're doing the right thing by clarifying your responsibilities. If you get an answer from your taxing authority and EPA that you feel provides you sufficient protection against fines and fees, get the source (tax code, law etc) printed out and keep it with your on-road vehicle. Being able to show a policing authority written justification for your actions may mean the difference between getting a hefty ticket if you get your tank dipped. If it's written down and shared with them in a respectful manner, you won't appear to be a teenager caught with his pants down who thinks he's smarter than everyone else.
Good luck in your efforts, and for the sake of other enthusiasts in your state, please report your findings back to this thread!