Hi mastermixer,
Plywood would be OK under a foundation but I would tend to use caution with plywood or any non breathable surface directly under the mattress unless there were no other good options (see post #10 here).
I would also add some slats under the bunkie board as well for better support such as the Ikea slats here that are designed to be used with their beds. Some bunkie boards are built stronger than others (many are meant for use with bunkbeds and children rather than as a thinner replacement for a foundation) and I would also hesitate to use a bunkie board without any supporting slats underneath to give some strength to the load bearing surfaces of the bed.
Phoenix