I have not shot my 12 ga from atop the ladder, but I would not hesitate to do so if the opportunity presented itself. After sitting on the ladder for a few minutes and figuring out how to balance I don't think shooting the shotgun will be problematic. I think I would try to get balanced, lean into the gun, and anticipate the recoil. Of course, this is all theoretical at this point as I haven't actually done it. It's quite possible that I will totally forget that I'm shooting the shotgun, pull the trigger before I'm ready, and go a$$ over tea kettle off the ladder, falling 5'+ to the earth below, causing grievous injury to my decrepit, arthritic body, and requiring Life Flight to medevac me to the nearest trauma center.... Hopefully that won't happen, but it is possible.
My best advice would be to try it with a buddy, in a controlled environment, in an area with a rapid response 911 system, and good cell phone coverage. Definitely not a
"Hold my beer and watch this!" kind of thing.
Let us know, and post pics!