Holy crap I got it working! I’m posting the solution here so hopefully the next person won’t have to spend 4 working days to solve a 30 second problem.
Instead of using the rigging/animation/bones in blender, I just used a parent/child relationship with the meshes themselves, so I had base, UpperArm, and ForeArm.
Then in the import to UE and check the “skeletal mesh” box and it automatically created bones/joints for each segment.
In the actor, add a poseable mesh component (as child to default scene root) and select your newly-imported thingy as the “Skeletal Mesh” and in blueprints you manipulate them with “Set Bone Rotation/Transform by Name”