Let me describe the solution to my own problem about "No accepted shares" in case it might benefit somebody whose search brings them to this thread.

I have two separate farms of gridseeds, but when I wanted to learn to use Raspberry Pi, I went to a third location.  I wanted just a standalone rig where Pi and I could get acquainted.  No hub, just USB straight from Pi to GS.  For power supply, I used a single-unit, "12v 1A" supply that came with one of my Gridseeds.  And that power supply is what turned out to be the problem.

As soon as the theory hit me that the power supply didnt have enough ass, I wanted to check the voltage under the load of the GS.  It has a molded plug, so no screwing off the plastic cover to get at the terminals.  If it had regular twin lead running to it, I could have stuck a needle in each wire and measured the voltage that way, but no, it's coaxial cable.  So I hooked up another power supply, and bam bam bam one acceptance after another.

It's a blessing, really, I learned lots more about the Pi than I would have...