MAO-A, the subtype of MAO that metabolize DMT, is found in neurons (brain, spinal cord, and the peripheral nerves), astrocytes (brain and spinal cord), the liver, the gastrointestinal tract and (in females) the placenta. What MAO-As that actually do the metabolizing of DMT is not known to me, but I would guess it is mainly in the gastrointestinal tract, the liver and also the brain.
I find it very unlikely that a time-release-DMT can be made with the knowledge we have today, BUT it has been proposed that the yuremamine present in Mimosa does exactly this, as it contains DMT within it's molecular structure. As you may know, Mimosa is in fact active orally, although in much larger doses than with a MAOI.