Welp, tried and tried.
Checked out a couple LBS mechanics and neither of them have worked on the nexus hubs. One even said he tried to stay away from them (at least taking them down to the innards). Harris Cyclery, where I bought the wheel online, assured me there was only one size dustcap (I still think it's too small!). They said just grease it up and don't worry about the dustcap.
Now the shifter cable is too short to reach my monkey bars! So I bought some longer cable today and hope to have it up and running soon. At least rode it up and down my hill a few times without the cable (stuck in 1st gear) and it seems pretty smooth. Maybe a little tall in the gearing with a 52 up front, 165mm crank arms and a 19 tooth in back. I'll have to take it somewhere flat once the shifter's all hooked up before I get a bigger cog.
Thanks for the help all supplied here on my first thread. Bummed I couldn't figure it out, though.