Ben, I remember seeing a few handlebar setups that were based on a stoker stem in the DIY handlebar thread. IIRC, usually the base of the stoker stem is used with another piece of tubing or a quill stem stuck into it. I don’t know that any adjustable stoker stem would be long enough to be used on it’s own.
I’ve considered getting a T-bar for my 29er but it seems like UDC has been out of stock for a while.