The simple fact of the matter is that Freeloader and the like are not supported by Nintendo. Installing unsupported addons is at the risk of the console owner - caveat emptor (buyer beware for those that don't speak latin). If an update Nintendo release happens to cause an unsupported addon to stop working, that's soley the problem of the person that installed the addon.
There's nothing stopping you installing these things, but it's not Nintendo's problem if it breaks.
As to whether Nintendo can force upgrades to your Wii, this is covered in the Wii Network Service Privacy Policy, which you would have had to agree to when first connecting to the network:
"To ensure that your Wii console is operating properly and efficiently, we will from time to time automatically download updates, patches, upgrades and similar software to your Wii console. "
18/06/2008
Super Smash Bros. Brawl
Canberra, ACT