Proposal text: may be found by opening the below spoiler.
Argument: The challenged proposal states that "All labels must use the customary measurement units of that WA state." However, Article 3 of GA#88 "ENABLES member nations with the freedom to determine if they shall prohibit their private enterprises or any non-plenipotentiary citizens of their nation from using any units of measurement or variations of numeration or mathematical notation."
"All labels" includes labels which private food packagers - restaurants, supermarkets, wholesalers, and purveyors of cottage foods - affix onto their foods. If such labels use units forbidden under GA#88, it is a violation of WA law, regardless of what this proposal says. Arguably managers of state-run food packagers may also face responsibility if they allow or authorise the food they package to be labelled with forbidden units.
GA#88's blocker is a broad provision allowing member states to forbid the use of any units in any context for any combination of reasons. This includes the use of units on food labels. A member state can ban the use of some units in some contexts while allowing or requiring them in others: it can require fuel to be measured in gallons by petrol stations, allow servings of beer to be measured either in pints or millilitres by pubs and bars, and require that bottles of olive oil be measured in litres.
Suppose that a certain family of non-IMI units, such as imperial units, are colloquially used by the general public in a member state to describe measurements. Those units are "the customary measurement units." That member is still well within its right under GA#88 to require that all measurements on food labels be indicated with IMI units instead of those units, such as for the purpose of international standardisation or to improve consumer understanding.
Pre-Packaged Foods Labelling would require that member to ensure that these non-IMI units are used in product packaging, even though GA#88 forbids the WA from making such decisions on behalf of member states. Under the status quo, a member state can require that a particular food be labelled as containing 5,000 kilojoules, while banning it from being labelled as containing 1,200 calories, even if the general public overwhelmingly prefers to say in conversation that this food contains 1,200kcal of energy rather than 5,000kJ. If this proposal passes, it would have no choice but to label that food as containing 1,200kcal of energy, irregardless of its wishes and powers.
Since Pre-Packaged Foods Labelling requires pre-packaged foods sold in member states to bear labels featuring "the customary measurement units of that WA state," while member states are allowed under prior and standing law to forbid the use of any units whatsoever (including "customary measurement units") on food packaging, Pre-Packaged Foods Labelling CONTRADICTS GA#88.