Invoking hook_custom_tokens and hook_custom_tokenValues in Joomla
Here's info about using CiviCRM hooks in Joomla:
https://wiki.civicrm.org/confluence/display/CRMDOC/Example+Joomla+Plugin+for+implementing+hooks

But why create a Joomla plugin if you can also use the hooks in a CiviCRM extension?
https://wiki.civicrm.org/confluence/display/CRMDOC/Create+a+Module+Extension

The advantages of using a CiviCRM extension over a joomla extension:


Works on all CMS'es
Much more knowledge available in CiviCRM community


The only advantage of Joomla plugins I see would be if you would need to combine CiviCRM functionality with Joomla functionality.