Looking up objects that point to other objects

Here's a couple of tasks that require brute-force with the Git object format that I've wanted to do at some point.

 * Associate a blob with trees

   Given a blob sha1 find trees that reference it.

 * Associate trees with commits / other trees.

   Given a tree find which commit points to that tree, or a parent
   tree N levels up the stack that a commit points to.

Has anyone written tools to do this? They'd obviously be very CPU and I/O intensive, but occasionally I encounter cases where I'd find this useful, e.g. to find what commit contains this huge blob, or what trees / commits are involved with a corrupted object.