Just pour it in the gas.
The reason for running an external tank with 2 stroke oil, is because from the factory the omp injects regular motor oil, so it should work a little better if you use 2 stroke oil, which is made to mix with fuel, get burned, etc. The tank idea seems like a waste though. Just pour it in the gas, and leave the stock omp too.
If you have an s5, it's a good idea to add a little 2 stroke oil to the gas anyway, because the electronic omps inject very little oil to keep emissions down.
Also, you will know when your omp goes bad. Car goes into limp mode, so I wouldn't worry too much about that.
LOL at dudes with na/kas talking about rotaries "blowing up".