Non-eyeletted rims were once simply known as washer rims. Some mfrs supply the washers and some don't. I started building wheels over 50 years ago so some of my knowledge might be regarded as simply historic trivia. But if historic trivia is also good engineering practice maybe use the washers. Guys I learned from and worked with would never even consider building a non-eyeletted rim without using washers. If some funky modern rim were shaped so that washers just wouldn't fit (this is not the case with your rim) the old guys would simply not build that rim.

Take the nearest empty rim and put one spoke and nipple through it. Move the spoke and nipple around, see how much angle the spoke will move through without binding or forcing. Now reassemble with a washer. It's like you just put a gimbal on that nipple. And the washer spreads the load over a larger area of rim. Spoke pullthrough is not very common. On a heavily loaded tandem with a light rim and no washers you can expect pullthrough.

Tandem falls are no fun. If you want to trust your safety and your loved one with a rim that is specifically not rated for even close to the load you carry you are free to do so. Not a good idea.