I don't think that it is explicitly stated anywhere that you do NOT
need to have a permanent copy of the NetBSD tree on the machine where
you run sup.  If you are using a service provider or whatever and disk
is too expensive to hold the whole ~110MB tree you can:

1) run sup
2) tar cvfz netbsd.tar.gz src (~24MB)
3) find src -type f -exec rm {} \; -print
4) download and install netbsd.tar.gz

This leaves me with 1MB of directories, I'm not sure if I can erase
them, but I plan on trying "mv src foo" and seeing if sup still works.

Once your over this hump future upgrades are a breeze.  Just repeat
the four steps above; the sizes will be much smaller.