As I remember, the early years of the GL1800 had ABS as an option. My 2005 had it. But the interconnection between the front and rear brakes was the same whether or not the bike had ABS.
Most riders make the change successfully from foot + handlebar to foot only in due time. After all, almost all of us have no problem with a foot brake only in our cars and trucks. So why should it be different with a Spyder? Beats me. If adding touring foot rests creates a situation where one has a real delay in moving from the foot rest to the brake pedal, then I will agree a handlebar brake would be good. In my experience and understanding I don't see how having a hand brake would improve the braking operation of the Spyder. The brakes are not only ABS equipped, but the brake computer distributes the braking force among all three wheels for the best stability of the bike. The computer applies more braking to the front wheels, because like you've said, that's where it's needed. In case you aren't aware the VSS (or Nanny) will automatically operate the brakes if you get into a situation of going around a curve too fast.
When I read Nexus' post above the question that came to my mind is, "Did repeatedly installing a hand brake on each new Spyder inadvertently reinforce his brain's autopilot mode for the continued use of the hand brake, or was there such a deep seated autopilot reaction of reaching for the hand brake that giving up a hand brake was impossible?"
As I see it, the issue of a handbrake is not an issue of what is needed to stop the the Spyder most quickly and safely, but rather an issue of reprogramming our brains. Reprogramming sometimes can be very hard.
To learn more about how our brains operate in autopilot mode watch the series "Hacking Your Mind" on PBS. Three of the four episodes can be streamed. The fourth one airs Wed, 9/30.
Of course, no way do I question the advisability or necessity of a hand brake for those with physical limitations. After all, cars can be fitted with hand brakes if needed.