joints
I'm not sure if this exists or not.
A flat square piece of plastic or metal which has two or more permanently affixed dowel-type protuberances that allow various boards to be joined simultaneously.
If it exists, would love to know the name so I can shop for them.
Buy a self centering dowel drilling jig. Lee Valley has them, as will virtually all wood working supply sources, Rockler, for example.
The jig clamps to the end of your board, and then you drill through a hardened drill guide into the board, using a hand drill. These dowel jigs used to be much more common in the days before biscuits, pocket screws, etc.
The simplest solution is to drill the dowel hole after you fit the joint together.
Fit the joint and make sure everything's square and true, then drill straight through the mortice and tenon in one go and insert the dowel. Chisel/plane and sand it flush and you should be good to go.
Making a mark on the outside of the mortice where you want to drill will ensure you get the location right.
Best Answer
Probably not.
However, you have an X Y Problem
Commonly used solutions include
Furniture bolts
Mitre bolts
Knock-down fittings
AKA furniture blocks
Cam lock bolts
Woodworking Joints
See https://woodworking.stackexchange.com/questions/3305/creating-a-speaker-stand-using-only-joins/3306#3306