Hey Betadog,
Your correct, right now this is not possible. It is possible to list a category from a post type, but not multiple post types, and not multiple categories.
We did not have this functionality planned, but it is a good idea, and I can see how it would be useful.
I will be working on this functionality for v. 1.3.5
In regards to the post_type parameter; were you thinking a comma separated list would work for the value? i.e. post_type=”books, magazines, VHS”?
Thanks,
Bryce