Using NvidiaInspector, I'm finding an option labeled "Enable Application for Optimus" near the very bottom of all the options; might be able to do something by adding a profile for Dolphin and fiddling with that...
AMD Ryzen 7 3700X @ 4.2 GHz | MSI GTX 1070TI 8GB | 32 GB DDR4 @ 3000 MHz | Windows 10 Pro x64 | Dolphin 5.0-10886