IP Masking means that we create a temporary account for any user that's editing without being logged in. More details on how temporary account work here, and more details on what to look for when adapting an extension to handle temporary accounts is here
A quick search for relevant code in the MultiMediaViewer extension is here
We'll need to consider whether to change isAnon and isRegistered, and test everything to make sure it still works when logged in as a temp user or a regular user or not logged in at all.
To enable temporary accounts in your local dev env, add this to LocalSettings.php
$wgAutoCreateTempUser['enabled'] = true;