That old SDK was kinda useless. There will be a new one though (iirc, MN1 said he's already working on it).
NUSGrabber can grab full titles. WUP Installer can download the package. It's just a matter of how long you're willing to wait, because the bloody thing never gives feedback. Not even after the download is done.
I think sig checks should be pretty easy. If you look at the update partitions found in almost every game disc, IOSU checks them if they are signed, and if they are, your firmware updates.
However, if we could exploit sig checks in the first place, none of the hassle would be necessary.
With an IOSU exploit in place, it should definitely possible. Perhaps we could run unsigned code from other spaces than the vWii, for a change? :3
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Possible, but nobody will actually take time to develop it.