I have the M8 and a Wh-200 (also have probably all the Wimberly flash bracket parts because most of my 500/4 shooting is with flash). I've never used the M8 for that purpose. I could post a quick pic if you are interested in seeing it put together.
There are at least two problems with this solution:
1. The only way to level the horizon is to re-level the legs. If you have a leveler between the WH-200 and the tripod then this might not be as much of a problem but I have never gone in that direction.
2. The M8 is not long enough to balance the camera on the gimbal...
Any camera with collarless lens, mounted on the gimbal, will cause the nose to drop *HARD* if you un-lock the gimbal pivot. You can look at that from the perspective that unlocking the gimbal pivot is just something you don't do when using it in this mode, but this has always bothered me.
I am a creature of habit, and my habit is to run my gimbal loose with a well balanced long lens. I also put in long hours, often dawn to dusk, out on the trail and at best tend to do stupid things as the day wears on . I think you can see where I am going with this .
A better solution might be a Kirk or RRS long plate with integrated clamp, similar to the M8 but longer. That would allow you to balance the camera so that it won't drop as hard. You probably still need to keep the pivot locked but this would avoid the hard drop in the event of a brain-lock.
Another option is to get a ball head like my M20 or M10, add a small plate to the bottom, and accomplish more or less the same thing that Michael (pompeio) suggests. That could also go on top of the long plate I mentioned in order to try to balance it.
I own multiple tripods, one usually with the gimbal and one always with a ballhead. There are times, though, that I would like to have a ballhead mounted on the gimbal and for that I plan on getting a plate some day, similar to the round plate RRS makes that bolts to a ballhead bottom. I think that would work on my M20? It would be easier, of course, to carry a ballhead on the trail than two tripods. One tripod with my 500/4 is about my limit
A ballhead on top of a gimbal is never going to be an ideal solution because at best it can not be as stable as a ballhead alone on the tripod. All the above is in the context of how to deal with occasional short focal length shots when out and about with a long lens and gimbal. To do it right you either need two tripods or do a lot of head swapping, as I tend to do.