In my rush to layout some Timbers for my friends who came over to help me cut joinery -- I drew the tenon in the post-top oriented in the wrong direction. And of course, they cut it the way I drew it. i.e. It is perpendicular to the mortice for which it is intended. What are my options for salvaging this joint, which is on a 20+ foot post that is otherwise finished and cut to length?

spline it.
this could be the way that splines where originally discovered???

Mark is right, your best bet to fix it with wood is definitely a free tenon.

What is the post joining at the top, a plate?

Gabel

Yes, the post is joining a plate above. Why?

Thanks Fellas.
Spline is a good idea -- never would have thought of it.

actually it's good to ask what the post joins to because the spline or free tenon may be working hard and therefore need to be harder wood.... If it's a 20 foot high post, then how big is the roof span and are there purlin plates to help out the post/plate connection? How long should the spline be?? Is there room for it(and it's pegs) down below?

Cutting back the post and adding a cogged half-lap scarf joint post extension with the proper tenon has historical precedent as a repair method.