As you note, this is not possible with out-of-the-box Jira; you would need a scripting add-on. As a work-around:
How do you know an issue belongs to a specific customer? Is it by the epic to which it is assigned?
If so, and you have just a few customers' epics to check, you can do this with JQL:
"Epic Link" IN (epicForCustomer1, epicForCustomer2)
ORDER BY "Epic Link", Key
If you have many customers' epics, consider instead:
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