Flying and an ostomy. No problems. I've been back and forth from UK to Canada 4 times in the last month. Not an issue.
My routine is to change my appliance the day before. The new bag will have taken by then.
Have 2 spare changes in your carry on luggage. I also have an extra t-shirt,and track pants. I've never used them,but more for piece of mind.
I always ask for an asile seat. I get up to streach and empty my appliance about 4 times,during the 6h flight.
Avoid caffine & orange juice. drink plenty water. If you want take 1 ammoidine pill to slow things down during the flight.
The biggest problem I had was going through the metal detector.It went off once and the guy gave me a body search. When he felt the bag,I got the what's this.I explain and there are more freaked out and embarresed than me. Hey who cares,I'll never see that person again in my life.
I sent away for a to Dansac for a "Dansac" Travel Certificate. This explains in 5 different lanugues that you have and Ostomy.I only use it if need.
As for the seat belt, I always wear it during the flight,but slightly loose. During take off & landings,as tight as it'll go. Always Safety First!