Well, I took 2 parcels to the local post office with Label 888 affixed, and after explaining things (I came prepared with printed copy of the relevant DMM section, as well as a version that makes it a bit easier to parse).
Long story short: all is well. Where my clerks had been getting hung up was fulfilling the $0.00 PVI requirement. One is not needed. Having the clerk round-date the parcel in a clearly conspicuous place on the front of the parcel fulfills the obligation proving that someone scrutinized the parcel.
When they scan in the Label 888, it will ask them for the zip code the parcel is going to, and that should be all that is needed.
I've checked both tracking numbers on the USPS website, and both accurately show progress.
ebay's "add tracking" function accepts the numbers without issue.
If anyone wants the specifics, in the event they get push back from USPS staff, I've provided my handout below, as well as a link to a printable PDF.
Looks like you can get a roll of 2,000 Label 888 for about $20 at the moment, which should keep people stocked for a good long time.
https://revenue-collector.com/misc/...888-2025.pdf