I've never heard of "whom" only being used for the indirect object. I've always learned that prescriptive standard grammars of English call for "whom" as the pronoun to use in any object position (direct, indirect, object of a preposition).

 

Here are a couple of good sites for prescriptive grammar rules that would confirm your usage:

   http://owl.english.purdue.edu/handouts/grammar/g_proncase.html

  And of course, there are lots of other sites out there to confirm your thoughts.

 

  Of course, that doesn't mean that I would necessarily use the who/whom distinction in the handout; in any case, I might collaborate with the English teacher to let him or her handle the who/whom issue in the English class while I teach Spanish object pronouns in my class (using the other English object pronouns as comparisons).

   The reason I treat who/whom outside of other topics and never use them as examples is because my own native English grammar never uses "whom." I think that I'm part of the generation who is right on the edge of acquiring it naturally or not (I'm 24). I had to learn it as a formal rule, much like I learn Spanish object pronouns. If the use of whom as an object pronoun doesn't come naturally to me, I don't expect it to be a natural part of my students' mental grammars. (Just as a side note, I'm not advocating one way or the other here whether who/whom should be taught at all -- just sharing my own experience.)

   Good luck with your handout! I'm sure your students will leave their foreign language classroom, like many other students, feeling that they learned more about English grammar there than in their English classes!

   Jed