Kenichi
Thanks,
What's probably happening is that the collisions are getting missed because of the type of motion that's feeding into the bones. What I suggest your rigger does is add selective sliding attachments in the areas where you're seeing the collisions not working so well.
What I'd do is select say this muscle first:
Then select the fat, and use Ziva Tools > Proximity Tools > Select Vertices. This will select only the vertices on the muscle that are directly adjacent to the inside of the fat.
Reselect the fat, make an attachment and convert it's mode to sliding.
Make it a bit weaker, say 105
Re simulate. It will just help the collisions resolve in that area. And seeing as you don't want the fat to ever pull away from the muscle, a sliding attachment is a good choice.
Do this for all the areas where you're seeing this problem
Let me know how that goes