Microwaves will go right through any microwave-safe material. Some ceramics aren't entirely microwave safe. You can test this by running the oven with the empty dish. If the dish warms up at all, don't use it in a microwave oven.
If the plastic is microwave safe, it's not going to internally heat, and it should be food safe at the temperatures encountered in the oven. So leeching won't be an issue.