FWIW I've used 5 different tires from 40 to 45 mm on a set of wheels with 30 mm (inside) rims. On those rims the tires measure 2 to 4 mm wider than their nominal sizes. The 40 mm Strada Bianca Pro and 44 mm Snoqualmie Pass Endurance are supple and virtually slick, the 40 mm Marathon Almotion and Marathon Efficiency are stiffer and have some tread depth, the 45 mm Cinturato Gravel H falls somewhere between.
I haven't noticed any problems but I don't think tires with high side blocks would work as expected on rims far from the design width.
