Why cant you aim the hole for the crankshaft counterweight...?
I don't know what the smallest size of brass tubing they offer is, but 1/16" ought to fit..?
You would probably need to make a tool to keep the tubing straight as you tap it or press it into the hole.
It can't be too much of a press fit...it's a tough call.
The rod recess is where I've aimed on other engines, but I felt that there was adequate meat to retain a press in tube.
Your idea to enter the backplate might be the most practical. If you could find some plastic tubing and the right way to weld the plastic in there...hmmm. I've got CA applicators that have pretty tough but flexible plastic necks.
The plastic extensions on WD40 might be about right........just some rambling thoughts.