Actually this shouldn't be that hard when you simply want files for the wup installer. I can try to quickly do it When I got a bit time again.
You just need to download the TMD, read the ContentID (reading counting count from
this + doing a little loop over
the contents), download the content file (
*cough*), and download the .h3 file when the content is hashed (
type & 0x2003). (+downloading the ticket & generating the .cert file).
Actually building a (G)UI would be the hardest (most time consuming) part.
I've made a improved version of the FileDownloder I linked, if someone is interested in it, I could post it.