That's hilarious, but also do-able if you have enough information on that person. Just use your own credit card to buy them the base-level membership. All you need, really, is their name, address, and email address. Use their real email address if you want them to have the extra fun of being thanked right away for becoming a member. Use the wrong email address if you want them to instead enjoy receiving the shooting magazine of their choice and especially getting their new membership card in the mail.
That said, the angry recipient would no doubt call the NRA to find out who paid for it. So, better to sign them up via postal mail and use a bank check for the dues.
I'm feeling like Dr. Evil, here. But it was your idea, so you're really to blame for how much this amuses me.
I figure we're somewhere north of $1200 minimum (maybe quite higher), basing it off of the $25 minimum for those that just said +1.
Check out the high-rolling contribution bundler over here!
Remember it isn't (just) the money. The NRA has influence for the same reason the AARP has influence. Actual human membership.
Also: donated