I don't know of any reasonable way to retrofit a quality self cancel system to the FJR.
This is one of those unfortunate topics that have a small number of zealots that tends to turn this topic to s#it PDQ.
I will post a few observations about self canceling turn signals.
First, I have had Hondas with this feature and it worked very good, they were not basic time/distance units, they had conditions that needed to be met. In ~15 years of riding with self canceling signals I can only recall two times they made a bad decision, but the situations were not normal and I was aware that the signals would cancel. If you are a manual canceler and never ever forget the signals you can still turn them off just like you always do. If you forget, the signal nanny will take care of it for you.
I often ride sweep and as such I see lots of turn signals left on. Lots, throughout the day almost every rider will miss the lights at least once. There are some serial offenders that ride all day with one or the other signal on. There are a large number of riders that I can predict when they will leave a light on(1) and a very small number of riders like Fred W that only leave a light on under extremely abnormal conditions. I'm not perfect but pretty good about turning off my signals. I have seen most riders forget the signals, with the riders making many, many more mistakes than would be made by smart self canceling signals.
I have observed traffic alter when they see a turn signal, in anticipation of the bike turning. Usually the anticipation is fine, but in some instances it has caused the traffic to make very unsafe decisions. I have seen bikes with the signal on and a car anticipating the bike to turn pass the bike and really SURPRISE the rider.
A rider that is turning without a turn signal will be observing traffic and (normally) make decisions in a way that having a turn signal on wouldn't make much difference. A rider not turning with a turn signal on continuously alerts traffic you are turning so they anticipate it. Then when you don't turn it can be a significant problem. IMO, you are probably better off not to signal (passive) than to falsely signal (active).
(1) Sometimes when cornering it becomes a busy activity due to traffic, distractions, obstacles, etc and when this happens there are large numbers of riders in the group that will leave signals on. This is the only time/way I see the reliable cancelers forget the signal and this is what gets me also.