Is it okay to have soup (broth-based) before lunch and dinner? I think the warmth would fill you up enough so you won't eat as much for those who need a fill.

That's a liquid, just as much as Water. There are restrictions on drinking before, during and after meals. Also, liquid doesn't stay in your pouch and therefore doesn't contribute to a full feeling for more than just the time it takes for it to pass on through the stoma.

liquids at meals can make the food that you eat pass through the stoma more quickly than they would otherwise and can actually reduce the feeling of fullness and leave you hungry sooner than normal.