Hi Paul, there is no function for automatically removing internal parts. The closest thing is probably the select visible objects script that Marco links to above - when triggered that will select objects that have some pixels being displayed in any view. It will select all objects that could have possibly been targeted by a click right then. If you rotate around a bit in the 3D view and trigger that several times it should get all the outer objects selected.

Once you have all the outer objects selected, you can use File > Export to write just those selected objects out, or also do a Select > Invert and then delete the interior ones.

Also see here for some general tips on reducing memory load for mesh export:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5322.2

- Michael