I think it would be useful to have a hook that runs before archiving a
subtree. I'm attaching two patches: one that includes a hook in the
archive process, and another (by way of an example) that adds a function
to that hook for the org-attach library. You can set the option
`org-attach-archive-delete' to a non-nil value to have org-attach delete
a subtree's attachments when you archive it.

Let me know what you think!

Eric