I regularly see Karen Darke riding about over Houndkirk up in the Peak District so with a certain amount of willpower it seems possible. Of course the people you're targeting probably aren't going to have that kind of drive to begin with.
Bikepacking trikes? I've seen something close to them in the form of a nice steel George Longstaff loaded with a Carradice picking over some off road stuff on a 400k audax. Then there's something like this beast
which went to the South Pole. Actually it managed it where many two wheel bikes couldn't showing where different machines have different strengths.
The red tape that you would have to overcome is in the form of safeguarding. Basically DBS checks all round and all that this entails. Even people without mental disabilities can be considered vulnerable to abuse and exploitation and you need to be able to demonstrate a) you're trying to prevent people likely to commit that abuse or exploitation from working with vulnerable individuals and b) your working practices protect both the customer and the workers.
As for hoops, yes it's health and safety. I know H&S has a bad rep particularly among those who like to do something dangerous on the spur of the moment. However, in reality it's something that almost all of us do subconsciously. You take a phone or a tracker in case you get in trouble. If you're out on your tod you might not take that drop off that you would if you were with your mates because there's no one you can rely on to help you if it goes wrong. You've assessed a risk and you've either mitigated it or ruled out an activity. Simple. There is, of course, an art to getting it down on paper. Fortunately general thinking about H&S has moved on and it involves a lot more about the likelihood of things happening as it does about the consequences of something going wrong. You don't have to entirely mitigate a risk, you just have to mitigate it until it's sufficiently unlikely that it's acceptable to proceed with the activity. You'll also need to be brutally honest with yourself about whether you, as an individual, can cope with something going wrong for a vulnerable person in your care.
Final first thought, don't go rushing off into the deep end. The most accomplished and hardened bikepackers out there didn't wake up one day from a quiet life, buy a bike and a stack of kit and go bash out the HT550. We've all been on a journey of testing and development. Facilitating that journey might be what you should be offering. Start with something like a trip round Derwent Resevoir before you're trying to get them up the Stanage Pole track before you try and get them over Cutthroat Bridge before you attempt Cut Gate before... You get the idea. You'll be learning how to do these things as they do.
