The Showa forks on the Tiger are pretty much perfect for things like touring and dirt roads. They may even be fine for fire roads if the pace is relaxed. But they are not sprung or damped for off-road.
I am 6' 1" and 220lbs. I took my Tiger down a whooped out two-track not long ago and even at a moderate pace (2nd gear, just loafing along) it was all the Tiger's front forks could handle. If I pushed the pace my enthusiasm was rewarded by a loud CLANK from the front end, the Tiger reminding me that it's not a dirt bike.
I'm used to a great deal of specialization from my motorcycles. I own a road racing machine, a SuperMoto bike, and a couple of very nice trail bikes for the woods. Each and every one of them has had a great deal of work done to the suspension (both ends). The Tiger is exactly the opposite idea for me: I want it to work well in all the environments into which I ride it.
If you go down that path with your BMW and find a good solution it would be great of you to come back and share it with us. We will be sure and do the same!