I've had a similar concern about doing crunches and other abdominal exercises after my cystectomy. I used to have great abs and a flat stomach. My surgeon doesn't think there's a time after which we're safe from hernias, and suggested I use a hernia belt, like weightlifters use.
Kegels are helpful if you have pelvic floor issues, regardless of whether you have a urostomy or neobladder. But I don't think they prevent hernias.