I decided to try this out. I put a plastic cutting board (about 2-3x thickness of otterbox) on top of both the jbl speaker and the charging plate. Both started charging pretty much as soon as it was on top. I'd say an otter box is a safe bet. I can't say if the charging performance is impeded (i.e., time to charge), but it did fire right up.
image:
https://skydrive.live.com/redir?resi...E06EB12D7!2195