I was trying to use PLUGIN_PATH/get_plugin_http_path() in KTD but it was producing an invalid URL path. But that's because that variable/function dates from bug 7804 (written in 2010 and then added to Koha in 2013). Back then, I remember it was sometimes necessary to manually add 'Alias /plugin/ "/var/lib/koha/plugins/"' to your Apache configuration. It appears that Alias was added to "etc/koha-httpd.conf" in bug 7804 but not added to the Debian Apache files. These days, plugins are served from other places than "/var/lib/koha/plugins" anyway. But yeah... just noting this for now...