This Monday Charles Martinet will be racing against random gamers from Australia and New Zealand.  For a chance to test your skills against the voice of Mario, just enter the Australian / New Zealand community code (39-7327-4613-9024) into Mario Kart 7, then play in the community between 6:30pm(AEST) and 7:30pm(AEST) on Monday, March 19.  If you get matched up with Martinet, his Mii will appear in your Mii Plaza -- assuming you've enabled the Invite Mii option in the Mii Plaza settings.

For anyone interested who lives outside the Australia and New Zealand portion of the world, it may be worth noting that community codes don't appear to region locked.

Links:
Nintendo of Australia Mario Kart 7 Community site
Nintendo World Report's article