Please tell me about smoke systems.
By the nature of the beast, you pretty much HAVE to put the oil in before the pressure tap, here's why:
The narrow part of the muffler causes the exhaust to flow fast, which creates a low pressure area. In the barrel (or expansion chamber) of the muffler, the flow is slower, but the pressure is greater. THIS is where a muffler is SUPPOSED to be tapped for pressure.
So if you have a nipple on the flex hose already, that's a perfect place to attach the oil line. Then, just tap the muffler body for fuel pressure.
I really don't think you will have a problem with fuel contamination. The minuscule amount of oil that gets in will only add to the oil that's already in the fuel.