The simple answer is that I always wear a seatbelt in the car. However, I do act as an escort on a 'welfare bus' where a few or our passengers have similar problems to those of ostomates and do not want to wear seatbelts.
Unfortunately, for insurance purposes there is a necessity for everyone on the buses to wear seatbelts at all times. In these cases we have come up with several ways to make the WEARING OF SEATBELTS MORE COMFORTABLE.
1) A simple clothes peg, clipped on the belt at the place where it comes from the shoulder pivot, stops the belt from gradually getting tighter. ( The belt will still work if there was an accident.)
2) We requested that any new buses were designed differently, so that this problem could be overcome. Our new buses have the option of having the seabelts hang from a point above the person so that they do not press on the shoulder or the abdomen.
3) In the case of wheelchair users, the belts can be fixed so that they also hang from above the passenger, rather than over the shoulder. also the design is completely different and much more comfortable.
4) For extra large passengers, we have extentions to the fixing point near the seat. these have the effect of moving that point further forward at the lower abdomen, thereby relieving the pressure on that area.
5) I have bought foam covers for seatbelts, (available from motoring stores) which also takes the pressure off the body by spreading the load and making the belts more comfortable to wear.
6) This concept could easily be extended to incorporate a stoma guard into/upon a seatbelt so that the passenger would not be wearing it on their person and would only have a need for it when they are wearing the seatbelt.
7) For cars, there are a variety of alternative arrangements for replacing the conventional (cheap) seatbelts. (They are usually fitted on the more expensive vehicles.) However, we have a voluntary organisation of engineers in our area who will take up the challenge of making the lives of disabled people more comfortable and manageable. I am sure that if someone presented them with a problem such as this, they would be only to happy to address their collective minds and skills to overcoming the problem.
(My own suggestion for protecting stomas, would be to use the concept of an extention to the lower fixing point and design it that so that it is a stiff curve to the shape of the body at the area of the lower abdomen where the stoma is situated. The shoulder strap would then be fixed at a point above the stoma and the device should act as a stoma guard.) I think that my suggestion would need to be tailor-made for each person, as stomas tend to be in different places. However, once the principle is established, this should not be too difficult for a mechanic or engineer to make.
I hope my comments are helpful in finding solutions to what should be a relatively simple (mechanical) problem.