I bought some of the Fimo at Michaels and tried it today on one of Steve's Oval keychain fobs. It's drying from its lemon oil bath so I haven't put any spray on it yet. As best as I can tell, it worked fine.
The only thing I'd mention is that the clay dust when you sand gets all over the lighter wood. Eventually I just bit the bullet and used a damp paper towel on it, not knowing if it would make it better or worse. Fortunately, it made it better.
That might be normal with all the inlay materials. Today was just the first time I've tried anything remotely like this.
If there's a better solution than the damp paper towel, someone please let me know. I've got the vast majority of this stuff left. I used less than 1/16th of it for the fob and I made sure to really overfill.