I'm wondering about the freewheeling driveshaft and stress on other parts like frame, transmission, hub, tires, triple tree, etc.
What are some thoughts from this esteemed brain trust?
I have no experience with the product you named but 40 years ago I have a bracket ( piece of "U"Channel) that i strapped the front wheel into that bolted to the tow hitch on my car.
The bike in question was an XL350 Honda.
It towed really well for the 40 odd miles it took for the normally pressure fed bearing in the gearbox to seize, after that no so much.
I had simply copied what a friend had made for the their 2-stroke trail bike which didn't have any pressure fed bearings int he gearbox.
After I fixed my gearbox I persevered with it but took the chain off prior to towing from then on, an annoying little job but as you rightly said, it beat the hell out of taking a full trailer.
In relation to doing this to a BMW airhead.
The "physics" of the towing will not hurt anything as the loads are going through the same components that they normally would, except that your weight will not be on the bike. The elevation of the front wheel isn't going to significantly change either the value of the forces involved or their direction.
There is one thing I don't know and will leave you to find out, which I'll get to shortly.
The final drive will be fine as it will be operating in its "normal" direction and oil will be distributed appropriately.
I am not so sure about the gearbox. Some of the gears fling oil up to the "roof" of the gearbox where some of it then collected in a piece of metal channel and then flow into oiling points for a number of the bearings (it is this bit of channel that drops and causes all the trouble if you foolishly undo that tempting screw you see when you remove the airbox). I am unsure that the bearings at the front of the gearbox would receive their ration of oil with the front wheel essentially elevated into a "wheel stand" position long term - oil like water will not flow uphill.
BUT, plenty of gearbox rebuilders have taken to rebuilding using sealed bearings (they are much easier to obtain in some sizes) and the world is not littered with failed airhead gearboxes as a result. Note that "sealed" does not mean hermetically sealed, oil still gets into them, just vastly less that in an open bearing, this tells me that the oil quantity requirements for the bearings are far, far less than the flood they receive from the piece of channel.
The next part of the equation is to determine which shafts are actually turning when the gearbox is in neutral but driven from the output shaft at road speed. I confess I have never turned my mind to working that out, you probably should as it will tell you which (if any) bearings are at risk from long term "free wheeling". I suspect that the requirements for lubrication of an unladen ball bearing are very slight and whilst my Honda XL350 experience haunts me, that was the failure of a pressure fed bearing, I should also say that 40 odd years ago when I was repairing my Honda gearbox at least one mechanic was adamant that the failure had nothing to do with being towed and that it would have happened anyway if I'd been riding the bike.
I confess I was less into contemplative analysis then than I am now, then I had a broken bike I needed running, so I just got on and fixed it, the fine grained detail of why it broke didn't really concern me too much.