Switched off my standalone companion plugin to play the game a bit.
Was picking up some equipment from a storage chest; went to move it to a CM companion... and it wasn't there.
Loaded another game, tried again. Disappeared again. Only items with an armor value - no other items had a problem.
Load yet another game, try taking armor off a companion. Two pieces disappear. Once those two pieces are sacrificed, items transfer back and forth again normally.
I wasn't running any new or modified plugins. Hadn't saved with the standalone companion enabled. It was the exact same game that it was the last time I played - when all worked correctly.
After accepting the loss of the cuirass and greaves, everything went back to normal.
I have no idea what happened.
I have got to get another hobby. This is getting too random even for me.
It is a sad, sad day when Fallout 3 modding is the more predictable option.
I wonder how hard it would be to turn one of the women in Dragon Age into a hot redhead...?