Ok, I found the button that Target (my apologies to him) mentioned. It seems I, like most here, just hit the begin buttons and start the battle. However, a post by Ii Naomasa at http://www.totalwar.org/ubb/Forum1/HTML/001973.html made me go look again. Unfortunately, I could only check easily as an attacker among my save games.
Right after you select weather and you can choose one of the army formations nobody seems to use , there is a new button on the top row on the left hand side. If you select a unit you want to get rid of and hit this cycle reinforcements button, it will begin substituting extra units from your reserve into your onfield army.
This doesn't solve the problems of no reinforcements coming onto the battle, but at least you get to pick the 16 units you want to start with.
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