Okay, reality check time.
Nobody really knows for sure what the Wii's USB hardware is capable of. It's a custom controller, and built inside of the Hollywood / Starlet, so we can't exactly look up a datasheet for it.
The IOS driver for the USB ports is called "oh0"/"oh1". This suggests the controller is
OHCI, implying USB 1.1.
There is a list of modules in IOS35 that reads: kernel, eticket, fs, di, oh0, oh1, ehci, sdi, usbeth, net, wd, wl, kd, ncd, stm, ppcboot, ssl, usb, p2p. That's a list of possible modules; p2p and ehci are not actually included.
EHCI would imply USB2.0. However, it's not clear if this is actually possible on the Wii hardware; it's entirely possible that this is / was only supported on development units. Someone would have to try writing a driver from scratch for IOS, which is not something that is trivial to do.