I'm pretty sure you need actually access to the person's console to do that. You can purchase a number of Wii points directly on their system with your credit card and then download a game if you can do that all under the radar for gift giving purposes. Closest otherwise would be to buy a Wii points gift card.

I could be wrong though. It really seems like Nintendo's style to allow you to gift someone (via Friend Code) through their mail with a game or something, but keep in mind Nintendo probably wouldn't allow you to forcibly gift someone a game due to storage concerns on the recipients console (which they would need to have full control over).