Update from OP: this conundrum has been resolved. The library in question will have native JSON support once it's ported over from its predecessor, and using JS

Update from OP: this conundrum has been resolved. The library in question will have native JSON support once it's ported over from its predecessor, and using JSON effectively from a high-level language requires some way of distinguishing between "property not set" and "property set to a non-value," which the undefined-vs-null distinction is a perfect fit for. It turns out that both null and undefined can coexist in the current model without either requiring a concrete value to distinguish it from the other, so the initial conundrum was not a genuine problem. Thank you to who helped me work through this.