If it were mine ...
I would use butt joints and glue on the head board, no dowels T&G or biscuits. Just joint the edges to get them square and straight.... no easy task on boards that long.
Lacking a jointer, I would use my straight line rip jig, and lacking one of those, a hand plane a constant mating of the the edges to insure a good "gap free" joint. This won't be easy either way regardless of how you do it. The reason I'm recommending a large panel in a mortise is to prevent racking under "high stress" activities. IYKWIM.
OR you can also just use long angle brackets on the back side and screw the head board to the posts allowing for some season expansion in the screw holes. This would be less work than a mortise, but requires a drill press and some filing to elongate the screw holes.
Mortise and tenons on all the frame members as well OR you can use the common metal brackets that you push down to lock into position from Rockler or other bed hardware sources. They still require a small mortise to set the bracket flush... I donno?"
The Spalted Maple should be just great looking!
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)