I certainly wouldn't particularly surprised for him to do a voiceover role or narration.
I wouldn't be surprised if they got all of them in to do voiceovers. They could leave it late in the day before even asking them. I believe they didn't always ask John Lesson if he was free until the final dubs were due!
Also Big Finish are working with AudioGo next year to do an 11 part Destiny Of The Doctor series, so it will be interesting to see who they get for the 9th release. Its the first post era 9th Doctor story regardless IIRC.