Ive not read all five pages, but did the op answer if the hi viz is at work or at home/commute?
If its at work, someone does an RA for the task, says hi viz may help with safety. Work provides the safety equipment. Operatives trial it, then management review, report, make decision, which could be compulsory. If its on the commute, have they provided the bike (bike to work scheme?) Then strickly speaking they could provide rules for this also. Company car users may have to accept rules to drive by or loose the car, even out of hours. You may not be being paid, but its their property your thrashing around on/in. If not either of these, they could suggest you do this, as they do want you at work rather than hospital, but its then your choice, doubt its enforcable.