You nailed it! I used either Rufus, or WinToUSB to install a Windows .iso to the storage card, I have forgotten which tool I used. The storage card is recognized by the computer as a regular USB drive, so I just set the boot priority to boot USB first. Plug it in and turn the computer on to boot Windows, unplug it and turn the computer on to boot Linux.
I have tried WindowsToGo on other PCs, but it was always too slow to be usable even with high-quality flash drives. The Framework storage cards are very fast (1GB/s read), and the laptop's bus lanes & southbridge do not create a bottleneck, so the OS feels just as responsive as if it were running from the internal SSD for regular desktop usage. However, you should run intensive storage tasks while booted to the internal SSD if you're planning to dual boot this way, because a NVMe SSD is still several times faster than the storage expansion cards.