Previously logged as an idea directly under the D365 Finance category - https://experience.dynamics.com/ideas/idea/?ideaid=33f8c142-9af2-ec11-b5cf-0003ff45834c - MS support suggested it's appropriate here. Within F&O, the Document template form allows custom templates to be uploaded for any LedgerJournalTable menu item (GL, vendor invoice journal, vendor payment journal, etc.). The UI gives the impression it's possible to have multiple distinct options, but they'll all point to the single customer/override template.

The extension to provide a custom Template ID which retains the standard system template - https://learn.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/user-interface/add-templates-open-lines-excel-menu - is relatively small, but given the UI appears to allow multiple templates, it would be nice to have this ability back.