Guestbook plugin two times same installation

I’m trying to use the guestbook plugin twice in the same grav installation with different message.yaml files. So far, I have edited the frontmatter where the save path to the messages.yaml is stored.
However, the filename for loading the guestbook entries is retrieved from guestbook.yaml in the plugins folder. Is there any method to have the guestbook.php file use the frontmatter variable instead?
The following did NOT work (in guestbook.php):
$filename = DATA_DIR . 'guestbook/' . $this->grav['page']->get('form/process/save/filename', null, '/');
This did not work either (in guestbook.yaml):
filename: <%= %>