Hi,
The content.prepare trigger is called only for the custom fields of the type "WYSIWYG". So if your custom item field is of that type, and what they say is true, then it will work.
For other types of custom fields, we didn't judge appropriate to call content.prepare for them.
Note however that custom item fields values are only displayed in the email notifications, invoice, order.
If your goal is to add something to where the input field of the custom field is being displayed (the product page on the frontend) then that won't help.