Updating here since it’s tax time yet again. We ended up “self-filing” using H&R Block Expat this year (it’s essentially an online form that takes you through a multi-part questionnaire and completes relevant tax forms for you). We reported our KiwiSavers as pension funds so they were included as foreign assets on form 8938 and FBAR. End of story. I honestly think this is a completely appropriate way of reporting KiwiSaver accounts given the US/NZ tax treaty.
Anyway, we filed our federal return electronically last weekend, it was accepted by the IRS on Monday, and our refund was deposited in our bank account on Friday (the refund was due to itemized deductions—nothing related to our foreign accounts). Hopefully this info might save others from entering the deep and scary rabbit hole that opens up when you google how to report KiwiSaver funds on US tax returns!