The actual mochi part is a total cinch. It's the stuffing-it-or-otherwise-making-it-interesting part that's a little more difficult.
I ended up quitting and rolling them in coconut (and then a little more rice flour to help them keep their shape). See the plain-looking ones in the upper right? Those ones are stuffed with a little glob of peanut butter and three chocolate chips.
I used a recipe I found online, but I fudged it up. But the way I did it is awesome, so I'm calling it mine and sharing it with you.
1 cup glutinous rice flour (plus extra for rolling)
1/2 cup sugar
1 cup water
Whisk together the rice flour and sugar in a microwave-safe bowl. Add the water and whisk until smooth. (It's going to be very thin, but fear not!) Cover the bowl with plastic wrap. Microwave on high for 4 minutes.
It'll be hot, so let it cool a bit before handling (and watch out when you take the plastic wrap off). If filling, scoop some out, roll it in the extra rice flour, and pat it flat. Put your filling in and pinch closed. Roll with more rice flour as necessary and shake off the excess. If not filling, scoop some out and roll in whatever you're gonna roll it in.
Now I'm going to go eat those peanut butter & chocolate ones...