Ok, let's get this straight:
A SD exploit is not possible, even with a SD-to-USB adapter for obvoious reasons.
An USB exploit is possible, but not many games make use of an USB.
The only thing left is a Bluetooth exploit, with things such as games with NFC readers (Skylanders perhaps) and things that can be transfered via the Wiimote (maybe corruped Mii's).
We'll just have to wait, because the Wii Mini hacking scene is thinking very hard on a possible exploit. We thought of things like backup discs, but the Trucha bug is patched on the Wii Mini.