Admin List Participants – add custom action to ‘with selected’ dropdown?

I would like to add a custom action to the list of actions to be done ‘with selected’ records. Basically I want to be able to set a specific field to a specific value, just as is done with the ‘approve’ action, which sets the Approved field to ‘yes’.

I can see that this possibility is allowed for in the PDb_admin_list class, specifically in with_selected.php and process.php. However, I have no idea how to do this in practice. Presumably it would have to be done in a custom plugin or in functions.php.

Has someone already done this? If not, can you give me some guidance as to how to go about it? I’m not sure I’ll be competent enough, but I’m happy to have a go.