Invisible post construction wood joint?

Sambostar

This guitar rack I'm making from a table, I need to add an additional support beam to the uprights but don't want to break the dowels hold in the existing uprights and the crossmember.  I can't get dowels or a mortise in there as it's flush and won't open to accommodate them. 

Also don't want anything showing on the outside of the uprights, like screws, through dowels or cabinet fixings. 

My options are pocket hole dowels or screws from underneath or corner braces, recessed flush, bottom and side into a rout in the support beam ends. 

Is there anything else or other clever way of doing it so it only shows from underneath?

the_jaffa

I've no real idea how feasible this could be but:

Could you use a threaded bar with washers and nuts on either end that can be adjusted out to provide the support then clad the bar in wood to disguise it.  The bar would be the support so the wood wouldn't have to be structural and could just be glued to itself around the bar.