I just tested mounting the package at /etp and it works fine.
Jon, the way to debug this is to look in your error log for the two messages that get written out by ETP's index.vuh for each request. They'll tell you what is the URL of the actual template that ETP is going to serve.
If you don't see a link that says "Edit this page", then either a) the page it's serving is using a master template that doesn't display the result of etp::get_etp_link, or b) you don't have admin permission on the package and therefore etp::get_etp_link doesn't generate the link.
