What is your goal with an S&S coupled bike? Is it just for splitting a bike in half to make it easier to put inside a car? Or take onto a train?
The Amtrak policy on folding bikes is clear that they have to fold. If you can convince Amtrak that your coupled bike also deserves to be considered a folder, then it should work but you would likely have different Amtrak employees each time.
Do you really see inexpensive used S&S coupled bikes out there? I do not recall how much extra my S&S bike frame cost to get it from the manufacturer with the couplers, but it was not inexpensive. If you really see a used S&S coupled bike in your size at a good price, go for it.
I do not know how important it is to use the special S&S grease on their couplers. I bought the grease and have been VERY frugal with it because it is not cheap. Maybe you can get by with cheap grease?
I see no reason that you could not split the frame on a frequent basis.
If you get an S&S coupled bike, carry a S&S wrench all the time. A few years ago one of my couplers started to loosen on a long ride and at that time I did not carry the wrench, so I had to stop a few times to try to tighten it by hand.
I used a mountain bike inner tube to make some sleeves for each end of my S&S "nuts" to keep the dirt and grime out of the coupler threads. If you ride on gravel, you may consider that, see photo below.
The downtube coupler gets a lot of road grime thrown up on it.
And, if you do this, you might want to carry some worn out socks so that you can slide a sock over the side of the coupler that has exposed threads for when you travel with the frame split. And maybe some rubber bands to keep the socks in place.
If your weight rating on the Bike Friday is on the frame, not any of the components, maybe buy a new frame and move the components over to it? I have no clue what a used Bike Friday frame is worth, but you should be able to get something for the old frame.
I bought my S&S coupled bike frame to save on airline oversize fees. The couplers and the case paid for themselves, but soon after my last flight where they did pay for themselves, American and Delta canceled their oversize fees for bikes. United followed soon after. S&S couplers are not as popular now that they do not offer as much in savings.
And if you are thinking of flying to Europe with an S&S coupled bike, you need a case for it. And it takes a lot of time and work to turn this (next photo) into the assembled bike (in the photo after that).
The above photo is most of my bike in the S&S Backpack Case. The rear rack and a few other parts went into my other checked luggage bag.
Another option are Ritchey Break Away frames, but I think if you are talking about a heavily loaded bike, I think the S&S couplers are stronger than the Ritchey downtube coupler.