I've had a look about and it doesn't that easy. Well, not for me.
Each device has a unique code for it's wifi card.
Each version of windows mobile 'probably' stores this in a different
location in the registry.
To write a dll for Basic4ppc, it would have to check the version of windows
of the device 'and' find the unique ID of the wifi card installed.
It 'is' possible, obviously, as you know.
But I think a Shell call to an already written app seems to me like the best option.
I have found a single .exe wifi toggle app if you wan't a link to it.