Thanks Ward!
Changing the in and out values helped greatly, but the mesh still twisted at the elbow joint – which was solved by deselecting ‘B-Bone Rest’. (What is this button good for, anyway?)
honestly i’ve never even noticed that button before, so i couldn’t say. but if it works, then go for it 
one thing i noticed in the BBB rig was that the DEF forearm bone (may be named differently; i’m going from memory here) was detached from (as in not parented to) the DEF upper arm. this is not a problem because they’re both set to follow the control bones. i detached it on one of my rigs and it cleared up some weirdness in the forearm/elbow area when it was bending past 90°. you might try that if you’re doing the same type of setup.
oops repost.