Ah yeah, that sounds like a shortcoming of the virtiofs driver for Windows then…
So while you can’t easily set that config option through the raw config keys, you may be able to use raw.qemu to define an additional network interface and then have that provide the SMB server. So basically providing a NIC using QEMU’s user driver or something like that, just to provide the SMB access, but then keep using the normal NIC for internet access.