Tom,
If I understand your situation correctly, a good use of that material would be to build several submerged "islands" in that real deep portion of the pond. Build the islands well up above where the thermocline will run and then add some permanent structure on it to top out just below the surface. I did this in a 4 acre pond with 25 foot depths. I wanted to make use of some of that "bad" water area in the deep portions and make it an asset rather than a liability for fish. It has worked really well. Those "islands" have turned out to be real fish magnets. It's relatively easy to build them, especially since you are moving the dirt anyway.
Regarding the rye grass, you probably know that you need to seed it really heavy for erosion control. It has a thin blade and tiny root system and it takes a lot of seed to hold soil. One thing that has worked for me it to use old hay spread around over the exposed surface area. It gives good protection, decomposes into good soil builder, and can provide a lot of grass seeds as a free benefit. I've used the big round bales for that purpose and just unrolled them over the exposed areas. Really stops erosion, but you need a tractor to handle them.
p.s. I thought you were in Guam, but see your pond is an Okie...yes bahia probably would not survive your winters.
